Measles is a very contagious viral infection that usually causes fever, cough, runny nose, red eyes, and a spreading rash. It most often affects children who are not vaccinated, but measles can happen at any age. Many people first hear about measles when early symptoms of measles seem like a bad cold for a few days before the rash appears. The illness usually lasts about 1 to 2 weeks, but some people develop serious problems such as pneumonia or brain swelling, and measles can sometimes be fatal. Treatment is mainly supportive with rest, fluids, fever relief, and medical care for complications, while vaccination is the best way to prevent measles.
How many people live with Measles?
Measles is a short-term infection, so health groups usually report how many cases happen over a year rather than how many people are living with it at one moment; worldwide, there were an estimated 10.3 million measles cases in 2023 (WHO and CDC, 2024). In the United States, 285 cases were reported in 2024 (CDC, 2025), so the number of people who have measles at any one time is usually much lower and changes quickly.
Overview
MeaslesSymptoms
Measles usually starts with fever, cough, runny nose, and red, watery eyes, followed by a spreading rash. Early symptoms of measles can feel like a bad cold at first, then the rash and fever often become more intense.
Outlook and prognosis
Most people with measles recover fully within 2 to 3 weeks. The outlook is usually good with rest and fluids, but babies, pregnant people, and anyone with weak immunity have a higher risk of serious lung or brain complications.
Causes and risk factors
Measles is caused by the measles virus. Risk rises with no vaccination, close contact, travel to outbreak areas, crowded indoor settings, and weak immune defenses. Babies, pregnant people, and those with vitamin A deficiency face higher risk of severe illness.
Genetic influences
Genetics play little role in whether someone gets measles; exposure and vaccination matter most. Rare immune-system gene changes can affect how severely measles affects some people, but they are not a common cause.
Diagnosis
Measles is often diagnosed from the pattern of symptoms, the rash, and recent exposure. To confirm the diagnosis of measles, doctors may use a swab from the nose or throat, or a blood test.
Treatment and drugs
Treatment for measles focuses on rest, fluids, and fever relief, while watching closely for breathing problems, dehydration, or ear infections. Many with measles also need vitamin A, especially children. Antibiotics help only if a bacterial infection develops.
Symptoms
Measles usually starts like a bad cold with fever, then a rash appears a few days later. Many people with measles feel wiped out, lose their appetite, and want to rest more than usual. The early symptoms of measles can be easy to mistake for another viral illness at first, especially before the rash shows up. Symptoms vary from person to person and can change over time.
Fever is often one of the first signs and can become quite high. For many, this means feeling weak, achy, sweaty, or too tired for normal daily routines.
A dry, persistent cough is common and can be tiring, especially at night. It may come with a sore throat and make eating, talking, or sleeping less comfortable.
A stuffy or runny nose often shows up early and can look much like a common cold. Sneezing and constant nose wiping may make the first days of measles seem mild before other symptoms build.
The eyes may become red, watery, and irritated. Bright light can feel uncomfortable, so some people prefer dim rooms or want to keep their eyes closed more often.
How people usually first notice Measles?
Measles usually first shows up like a bad viral illness: a high fever, tiredness, cough, runny nose, and red, watery eyes often start several days before the rash appears. One of the first signs of measles that doctors may look for is tiny blue-white spots inside the mouth, called Koplik spots, although many people notice the illness first because the fever keeps rising and the child or adult seems much more unwell than with a simple cold. A few days later, the classic red rash begins on the face, often near the hairline, and then spreads downward over the body, which is often how measles is first recognized by families.
Types of Measles
Measles does not look exactly the same in every case, even though the classic pattern is well known. In practice, doctors describe a few recognized types of measles based on how the illness appears and whether someone has some existing immunity. Symptoms don’t always look the same for everyone. For many living with measles, the main differences are how severe the fever and rash are, when the rash appears, and whether unusual warning signs develop.
Classic measles
This is the usual form of measles, with high fever, cough, runny nose, red eyes, and then a spreading rash. Small white spots inside the mouth can appear early, often a day or two before the rash. When people ask about the types of measles, this is the form most often meant.
Modified measles
This form can happen in people who have some partial protection, such as after vaccination or immune treatment, but not enough to fully prevent infection. Symptoms are often milder, with a lower fever, a lighter rash, or a shorter illness. Because it can look less typical, it may be harder to recognize at first.
Atypical measles
This uncommon form was mainly seen in people who had received an older killed measles vaccine used decades ago. The rash may start on the hands and feet before moving inward, which is different from classic measles. Fever, body aches, headache, and lung symptoms can be more pronounced.
Hemorrhagic measles
This is a rare and very severe form in which measles is accompanied by bleeding into the skin or from the mouth, nose, or other areas. People may look much sicker, and the illness can worsen quickly. This needs urgent medical care because it can be life-threatening.
Can I slow down the onset of symptoms?
Usually no: once measles has taken hold, there is no proven lifestyle change, diet, sleep plan, or home treatment that can meaningfully slow when symptoms begin. The main medical option is right after exposure, not later: for people who are not fully protected, getting the measles, mumps, and rubella (MMR) vaccine within 72 hours, or a protective antibody treatment within 6 days in certain high-risk groups, may lower the chance of getting sick or make the illness less severe, but it does not guarantee prevention. If you think you were exposed, getting medical advice quickly can still help, even though it cannot promise a different outcome.
What genetics can tell you about symptoms
For most people, genetics does not strongly predict which measles symptoms will show up, because the illness is mainly driven by the virus itself and by how the immune system responds at the time of infection. Even so, inherited differences in immune function can affect how well the body fights the virus, so some people may be more likely to become very ill or develop complications such as pneumonia or brain inflammation. Rare genetic immune disorders can also lead to unusually severe measles or symptoms that start with fewer typical warning signs, but this is not the usual pattern.
Causes and risk factors
Measles spreads through the air, and it can pass quickly in homes, schools, day cares, and other crowded indoor spaces. Your risk is highest if you have not had the measles vaccine or if you are around someone who is infected. Travel to places with outbreaks can also raise the chance of exposure. Babies who are too young for vaccination, pregnant people, and people with weakened immune systems can become much sicker from measles. Knowing the risks can help you feel more in control, and rare inherited immune problems may make severe measles more likely.
What are the most common causes or risk factors?
Measles is caused by the measles virus, which spreads very easily through the air when an infected person coughs, sneezes, or even breathes nearby. The biggest risk factor is not being vaccinated, and the risk is also higher for babies who are too young for the vaccine, people living in close quarters, and anyone around someone with measles at home, school, childcare, or on public transportation. Travel to places with active outbreaks or lower vaccination rates also raises the chance of getting measles.
Environmental and biological risk factors
Measles spreads very easily through the air, so risk often depends on both your immune protection and the places where you may be exposed. Some risks are carried inside the body, others come from the world around us. That is why who is at higher risk for measles can vary from one person to another.
People without immunity to measles are much more likely to get infected after exposure. This includes those who have not been vaccinated, have had only partial vaccination, or never had measles before.
Babies can be at higher risk once early protection passed from the mother begins to fade. Their chance of exposure rises if measles is circulating at home, during travel, or in group settings.
A weakened immune system can make it harder for the body to block the measles virus after exposure. This may happen with cancer treatment, transplant medicines, advanced HIV, or other serious immune problems.
Measles can stay in the air of an indoor space for up to 2 hours after an infected person leaves. Crowded homes, classrooms, shelters, clinics, and waiting rooms can make exposure more likely.
Genetic risk factors
Measles is not something you inherit in the way you inherit eye color or a single-gene disorder. Some risk factors are inherited through our genes. When people ask about genetic risk factors for measles, the strongest links are usually rare inherited immune problems and smaller gene differences that shape how the body recognizes viruses. These factors may affect how easily someone gets infected or how severe the illness becomes, but they do not cause measles on their own.
Inherited immune disorders: Rare inherited disorders that weaken key parts of the immune system can make measles much harder for the body to control. People living with these conditions may have a higher risk of severe infection, pneumonia, or illness that lasts longer than expected.
Recognition genes: Small differences in genes that help immune cells recognize the measles virus can influence how strongly the body responds. This is one reason inherited risk in measles is thought to affect severity and immune protection more than it acts like a single direct cause.
Interferon pathway: Rare changes in genes that control interferon, an early antiviral warning signal, can reduce the body's first line of defense against measles. When this pathway is impaired, the virus may spread more easily before the immune system catches up.
Family history: A family history of inherited immune conditions or unusually severe viral infections can be a clue that an underlying genetic issue is present. If several close relatives have had this pattern, doctors may look more closely for a primary immunodeficiency.
Lifestyle risk factors
Measles spreads very easily, so everyday behaviors that affect exposure, immune resilience, and early response can change a person’s risk of infection or severe illness. The main lifestyle risk factors for measles are being unvaccinated, spending time in high-exposure settings, delaying isolation when sick, and poor nutrition that weakens recovery. While habits alone do not create the virus, they can strongly influence who gets exposed and how severe the illness becomes.
Not receiving the recommended measles vaccine is the most important modifiable risk factor. Without vaccine protection, exposure is much more likely to lead to infection and further spread.
Frequent close contact in crowded indoor spaces, social gatherings, schools, dormitories, or during travel can raise the chance of catching measles. This matters especially when a person is unvaccinated or spending time in areas with active outbreaks.
Going to work, school, childcare, or social events while fever, cough, and rash are developing can expose many other people. Seeking care late and not isolating promptly can also increase spread within households and communities.
Undernutrition can make it harder for the body to cope with measles and may increase the risk of complications. Low vitamin A status is especially linked to more severe illness, particularly in children.
Risk prevention
Measles is highly contagious, so prevention mainly means avoiding infection before exposure and acting quickly after close contact. For most people, the strongest protection is vaccination, especially for children, travelers, and anyone without clear immunity. Prevention can mean both medical steps, like vaccines, and lifestyle steps, like avoiding exposure when outbreaks happen. If measles is circulating or you think you were exposed, early medical advice can lower the chance of serious illness and help limit spread to others.
The measles, mumps, and rubella vaccine is the best way to prevent measles and usually provides long-lasting protection. Keeping up with the recommended doses is especially important for children, teens, and adults who are not fully vaccinated.
Many adults are already protected because of past vaccination or prior infection, but some are not sure of their status. Knowing your risks can guide which preventive steps matter most, especially before travel, pregnancy, or work in health care or child care settings.
If you have been near someone with measles, contact a doctor quickly because treatment soon after exposure may help in some cases. This can include vaccination within a short window or immune globulin for people at higher risk, such as infants, pregnant people without immunity, or those with weakened immune systems.
Measles spreads through the air and can linger in a room even after the infected person has left. During an outbreak, avoiding close indoor contact with infected people can reduce the risk of getting measles.
Can you prevent Measles?
Yes, measles can usually be prevented, and vaccination is the best way to protect yourself and others. It also helps to avoid close contact with someone who has measles if you are not fully protected, especially during an outbreak. No step works 100% of the time, but being vaccinated greatly lowers the chance of getting sick or spreading it.
Transmission
Measles spreads through the air when someone with the infection breathes, coughs, or sneezes, and the virus can stay in the air in a room for up to 2 hours after they leave. It can also spread when virus droplets land on a surface and someone then touches their eyes, nose, or mouth, though airborne spread is the main reason how measles spreads so easily.
Measles is one of the most contagious infections in humans. If one person has measles, about 9 out of 10 people nearby who are not immune, usually because they have not been vaccinated or have never had measles before, may catch it.
When to test your genes
For measles, genetic testing is not used for early detection or to tailor routine care. What matters most is your vaccination history, symptoms, and whether you were exposed. Rarely, doctors may consider genetic evaluation if someone has unusually severe or repeated infections suggesting an underlying immune system condition.
Diagnosis
Measles is usually diagnosed when a fever and feeling unwell are followed by a cough, runny nose, red eyes, and then a spreading rash. It’s natural to worry when symptoms don’t have a clear explanation. Doctors often suspect measles from this pattern, then confirm the diagnosis of measles with testing, especially during an outbreak or after a known exposure.
Doctors ask when the fever started, when the rash appeared, and whether there has been contact with someone who may have had measles. This timeline matters because the early symptoms of measles often begin a few days before the rash shows up.
A clinician looks at the rash pattern and checks for common signs such as red eyes and small spots inside the mouth called Koplik spots. These findings can strongly suggest measles, although other viral illnesses can look similar at first.
A nose or throat swab can be tested for the measles virus, often using a molecular test that looks for viral genetic material. This is one of the main ways to confirm the diagnosis, especially early in the illness.
Blood tests can look for antibodies that the immune system makes in response to measles. These results may help confirm the infection, but timing matters because antibodies can take several days to become detectable.
How is Measles diagnosed?
Measles is often diagnosed based on the typical pattern of symptoms, such as high fever, cough, runny nose, red watery eyes, and a spreading rash, especially if there has been contact with someone who has measles or travel to an area where it is spreading. Doctors usually confirm it with a lab test, most often using a swab from the nose or throat, a urine sample, or a blood test that looks for signs the body is reacting to the virus. In some cases, the virus itself is studied more closely to track how it is spreading, but this is mainly for public health work rather than the main way most people are diagnosed.
Stages of Measles
Measles usually follows a fairly predictable course rather than changing in long-term stages. Often the process begins when you notice changes in daily life, such as fever, tiredness, and a child who suddenly seems much more unwell than with a common cold. The early symptoms of measles often appear several days before the rash. Most people recover fully, but some can become seriously ill, especially young children, pregnant people, and those with weakened immune systems.
Incubation stage
After exposure, measles is in the body but there are no symptoms yet. This stage usually lasts about 7 to 14 days, though it can be a little longer.
Early illness
Fever, cough, runny nose, and red, watery eyes usually begin first. Some may also develop tiny white spots inside the mouth, and people with measles are already contagious at this point.
Rash stage
A red or reddish-brown rash usually starts on the face and then spreads down the body. Fever often gets higher as the rash appears, and many living with measles feel at their sickest during this stage.
Recovery stage
The fever and rash gradually settle over several days if no complications develop. Energy can take a little time to return, and the cough may linger after the rash fades.
Outlook and prognosis
The outlook for measles is usually good in otherwise healthy children and adults who receive supportive care, with most people recovering completely within 2 to 3 weeks after the rash begins. However, complications such as pneumonia, dehydration, ear infections, acute encephalitis, and, rarely, the delayed brain disorder subacute sclerosing panencephalitis can cause long-term disability or death. Mortality is much higher in infants, pregnant people, adults older than 20, people who are malnourished, and those with weakened immune systems. In high-resource settings, death is uncommon, but globally measles still causes substantial mortality, especially where vaccination coverage and access to medical care are limited. Severe infection during pregnancy is also linked to miscarriage, premature birth, and low birth weight. Vaccination greatly improves prognosis at the population level by preventing both infection and its most dangerous complications.
Long-term effects
Most people with measles recover fully, but some can have lasting health problems after the infection, especially if the illness was severe or involved the lungs or brain. Long-term effects vary widely, from no ongoing issues to complications that can affect learning, hearing, vision, or overall health years later. Infants, adults, pregnant people, and anyone with a weakened immune system tend to face a higher risk of serious outcomes. Thinking about the long-term effects of measles helps explain why doctors take this infection so seriously, even though the early symptoms of measles can start like a fever and rash illness that seems straightforward at first.
In rare cases, measles can inflame the brain, a problem called encephalitis. This may leave lasting effects such as seizures, movement problems, learning difficulties, or changes in behavior and memory.
A very rare but severe brain disease called subacute sclerosing panencephalitis can appear years after measles, most often after infection in early childhood. It gradually damages the brain and is usually fatal.
Some people develop lasting hearing problems after severe measles, often related to ear infections or brain complications during the illness. For many, this can mean trouble following conversations at school, work, or in noisy places.
Severe measles pneumonia can sometimes leave ongoing breathing problems, especially after a complicated hospital stay. Some may find that exercise tolerance stays lower than before for a long time.
How is it to live with Measles?
Living with measles can feel like being abruptly shut down by a high fever, deep fatigue, cough, red irritated eyes, and a rash that can make even light and normal daily routines hard to tolerate. For many, this means staying home, resting, drinking fluids, and avoiding school, work, childcare, and public spaces while the illness runs its course. Measles also affects the people around you because it spreads very easily through the air, so family members, classmates, coworkers, and especially babies, pregnant people, and anyone with a weakened immune system may be at risk if they are not protected.
Treatment and drugs
Measles is treated mainly with supportive care, because there is no antiviral drug that routinely cures the infection once it has started. Most people with measles need rest, plenty of fluids, and medicine to lower fever and ease aches, while doctors watch closely for dehydration, breathing problems, or other complications. In children, treatment often includes vitamin A because it can lower the risk of serious eye problems and other severe illness, especially when early symptoms of measles have progressed quickly. If measles leads to a bacterial infection such as an ear infection or pneumonia, antibiotics may be used for that complication, but they do not treat the measles virus itself. Supportive care can make a real difference in how you feel day to day, and people who are very unwell, pregnant, very young, or have weak immune systems may need hospital care.
Can you cure Measles?
There is no medicine that cures measles once someone has it. Treatment focuses on helping the body recover and preventing problems, with rest, fluids, fever medicine, and sometimes vitamin A or hospital care if symptoms are severe. Most people get better with supportive care, but measles can be serious, so medical help is important.
Non-drug treatment
Measles can leave people feeling wiped out, feverish, and sensitive to light, so non-drug care mainly focuses on comfort, fluids, and watching closely for complications. For many, treatment begins not at the pharmacy, but with rest, steady hydration, and simple steps that make breathing, eating, and sleeping easier. Good home support matters for children and adults alike, especially when families are wondering about how to care for measles at home. Because measles can sometimes lead to ear infections, pneumonia, or dehydration, supportive care should be paired with clear advice on when to seek medical help.
Extra sleep and quiet time help the body recover while fever and rash run their course. Many living with measles may find that light, noise, and normal activity feel more draining than usual.
Sips of water, oral rehydration drinks, broth, or ice pops can help prevent dehydration, especially if fever is high. You may need to offer small amounts often when swallowing feels uncomfortable or energy is low.
Lightweight clothing, a comfortably cool room, and lukewarm washcloths can help someone feel less miserable during fever. Avoid cold baths or over-bundling, which can make chills and discomfort worse.
Moist air from a humidifier or a steamy bathroom can soothe a dry cough and ease throat irritation. Keep the device clean to lower the chance of spreading germs or adding mold to the air.
Your genes may have an impact on how drugs work for you
For measles, treatment is mostly about comfort and support, like helping with fever, fluids, and vitamin A, rather than using a medicine that directly targets the virus. In general, people’s genes can change how fast the body breaks down some common medicines or how strongly side effects are felt, but this is not usually a major part of measles care. Because of that, genetic testing is not a routine part of treating measles, and doctors usually base treatment on age, symptoms, and how sick someone is.
Pharmacological treatments
Treatment for measles is mostly about easing symptoms and preventing complications while the infection runs its course. There is no routine drug that reliably cures measles itself, so care often focuses on fever, fluids, and close monitoring. Many living with measles may also need extra treatment if they develop problems such as dehydration, pneumonia, or an ear infection. Not everyone responds to the same medication in the same way.
Acetaminophen can help lower fever and ease aches, headache, and general discomfort during measles. It is commonly used in both children and adults, with dosing based on age and weight.
Ibuprofen is another option for fever and body aches in people with measles who can take anti-inflammatory pain relievers safely. It should be avoided in some situations, such as dehydration, kidney problems, or stomach ulcers, and aspirin is not used in children because of the risk of a rare but serious condition called Reye syndrome.
Vitamin A is often recommended for children with measles because it can lower the risk of serious eye problems and other complications. This is especially important in severe illness, in young children, and where vitamin A deficiency may be more likely.
Oral rehydration solutions and, if needed, IV fluids help replace water and salts lost through fever, poor intake, vomiting, or diarrhea. For many, this can mean the difference between managing at home and needing hospital care.
Genetic influences
Measles is caused by a virus, so whether someone gets sick usually depends much more on exposure and vaccination status than on inherited genes. Still, genetics can shape how the immune system responds, which may help explain why one child with measles has a typical fever and rash while another becomes much sicker with lung infection or, rarely, swelling in the brain. Family history is one of the strongest clues to a genetic influence. Even so, for measles, family history matters far less than being vaccinated, age, nutrition, and whether the immune system is weakened. Researchers have found that small inherited differences in immune-related genes may affect the risk of severe measles and how strongly the body responds to the measles vaccine, but this is not something doctors routinely test for in everyday care. For most families, preventing measles still comes down to vaccination rather than genetic testing.
Is Measles genetic?
Genetics play little role in whether someone gets measles; exposure and vaccination matter most. Rare immune-system gene changes can affect how severely measles affects some people, but they are not a common cause.
Having a genetic risk is not necessarily a diagnosis
Genetic variants are a normal part of being human and are common throughout the general population. Most do not directly affect health and simply reflect natural differences among people. For many genes, having a variant associated with a condition suggests a possible link rather than a certain diagnosis, especially when several genes and environmental factors are involved. In some situations, though, a single well-studied variant in a particular gene is enough to confirm a diagnosis.
Pharmacogenetics—how genetics influence drug effects
When someone has measles, care usually centers on rest, fluids, fever relief, and vitamin A rather than a drug that directly attacks the virus. Not every difference in response is genetic, but for measles, genes usually play a smaller role in treatment decisions than age, symptom severity, pregnancy, and whether the immune system is weakened. If you are wondering how genetics affect measles treatment, doctors do not routinely use genetic testing to choose care for most cases.
Genes can still matter indirectly, because some inherited immune system conditions can make measles more severe and may lead to closer monitoring in the hospital or faster treatment of complications. If measles leads to problems such as pneumonia or an ear infection, the medicines are usually chosen based on the complication, medication allergies, and overall health, not on measles-specific genetic results.
Interactions with other conditions
For many people, measles does not happen in isolation, because it can temporarily weaken the immune system and make it easier for other infections to take hold. This is one reason ear infections, pneumonia, and diarrhea can become more serious during or soon after measles, especially in young children, pregnant people, and anyone whose immune system is already under strain. Another important aspect is how it may link with other diseases. Someone recovering from measles, for example, may seem to be getting better and then develop a new fever, cough, or trouble breathing from a second infection rather than from the rash itself. People with measles who are also malnourished or low in vitamin A often face a higher risk of severe illness, including eye problems and lung infections. If you are wondering whether measles can make other illnesses worse, the answer is yes in some cases, so new or worsening symptoms should be checked by a doctor promptly.
How genes can cause diseases
Humans have more than 20,000 genes, each carrying out one or a few specific functions in the body. One gene instructs the body to digest lactose from milk, another tells the body how to build strong bones and another prevents the body's cells from beginning to multiply uncontrollably and developing into cancer. As all of these genes combined are the building instructions for our body, a defect in one of these genes can have severe health consequences.
Through decades of genetic research, we know the genetic code of any healthy/functional human gene. We have also identified that in certain positions on a gene, some individuals may have a different genetic letter from the one you have. We call these hotspots "Genetic Variations" or "Variants" in short. In many cases, studies have been able to show that having the genetic letter "G" in the position makes you healthy, but having the letter "A" in the same position disrupts the gene function and causes a disease. Genopedia allows you to view these variants in genes and summarizes all that we know from scientific research, which genetic letters (Genotype) have good or bad consequences on your health or on your traits.
What other conditions can cause similar symptoms to Measles?
Several other illnesses can look like measles at first, especially rubella, roseola, scarlet fever, and some common viral infections that cause fever and a widespread rash. Allergic reactions or side effects from medicines can also cause a rash that may be mistaken for measles, especially if it appears along with feeling unwell. What often helps tell measles apart is the mix of high fever, cough, runny nose, red watery eyes, and then a rash that spreads from the face down the body.
History
For much of history, measles was a familiar childhood illness that could sweep through homes, schools, and whole towns in a matter of weeks. Before vaccination, many families knew what it looked like: a child with a high fever, cough, red eyes, and then a spreading rash a few days later. Across cultures and time periods, people described outbreaks that moved quickly and often hit many children at once.
As medical writing became more detailed, doctors learned to separate measles from other rash illnesses that can look similar, such as scarlet fever or rubella. Not every early description was complete, yet over time the picture became clearer, including the early symptoms of measles and the pattern of illness that often comes before the rash appears. By the 19th and early 20th centuries, measles was recognized as more than just a routine childhood sickness, because it could also lead to serious problems like pneumonia, brain inflammation, and death, especially in young children and in crowded or undernourished communities.
The biggest turning point in the history of measles came in the 20th century. Scientists confirmed that it is caused by a virus, and later a vaccine was developed and widely introduced in many countries during the 1960s. This changed daily life in a profound way: instead of expecting repeated school outbreaks, many communities saw measles cases fall sharply. In recent decades, awareness has grown that measles can return when vaccination rates drop, especially because the virus spreads so easily through the air. Today, measles is understood far better than it once was, but its history still matters because it shows how quickly a once-common infection can come back when protection in the community weakens.