West Nile fever is a mosquito-borne infection that often causes no symptoms, but some people develop fever, headache, body aches, tiredness, or a rash. The early symptoms of West Nile fever usually start 2 to 14 days after a mosquito bite and often improve within days to weeks. West Nile fever can affect people of any age, but older adults and people with weakened immune systems have a higher risk of severe illness. Not everyone will have the same experience. Treatment for West Nile fever is usually supportive, such as rest, fluids, and medicines to lower pain or fever, and severe cases can be life-threatening if the infection affects the brain or nervous system.
How many people live with West nile fever?
There is no widely used estimate of how many people are living with West Nile fever worldwide or in the United States, because it is usually a short-term infection rather than a lifelong condition. Public health agencies such as the World Health Organization and the Centers for Disease Control and Prevention mainly report West Nile fever as yearly or seasonal case counts, so a single current worldwide or U.S. “people living with it” number is not consistently reported.
Overview
West nile feverSymptoms
Most people with West Nile fever feel fine or have mild symptoms like fever, headache, body aches, nausea, or a rash. A few develop serious problems, including a stiff neck, confusion, weakness, or seizures.
Outlook and prognosis
Most people with West Nile fever recover fully within days to weeks. Tiredness and trouble concentrating can last longer, and older adults or people with severe brain or nerve involvement may face a slower recovery and lasting problems.
Causes and risk factors
West Nile fever is caused by West Nile virus, usually spread by infected mosquito bites. Risk rises in warm months, near standing water, and with outdoor exposure; severe illness is more likely in older adults and people with weakened immunity.
Genetic influences
Genetics may slightly influence how West Nile fever affects someone, including the chance of severe illness, but this is not the main driver. For most people, mosquito exposure, older age, and immune health matter much more than genetic variations.
Diagnosis
West Nile fever is diagnosed based on symptoms, recent mosquito exposure, and where you have traveled or live. Blood tests usually confirm the diagnosis of West Nile fever. In some cases, spinal fluid is tested.
Treatment and drugs
West Nile fever treatment focuses on rest, fluids, and pain or fever relief, since most people improve at home within days to weeks. If West Nile fever affects the brain or nerves, hospital care may include IV fluids, breathing support, and close monitoring.
Symptoms
Most people with West Nile fever feel nothing at all, but some develop a flu-like illness about 2 to 14 days after an infected mosquito bite. When symptoms do happen, they often include fever, headache, body aches, and tiredness, sometimes with a rash or stomach upset. Symptoms vary from person to person and can change over time. A small number of people become much more sick when the infection affects the brain, spinal cord, or nerves.
Many people infected with West Nile fever never feel sick. The infection may pass without any clear warning signs and only be found through testing.
Fever is common in mild West Nile fever and may come with chills or sweating. The early symptoms of West Nile fever can feel a lot like a short-lived summer flu.
Headache, muscle pain, and joint pain are common when West Nile fever causes symptoms. Day to day, it can feel like you are coming down with a virus and your whole body is sore.
Tiredness and a washed-out feeling can be strong even when the fever is not high. What once felt effortless can start to require more energy or focus.
How people usually first notice West nile fever?
Many people first notice West Nile fever after a recent mosquito bite when a few days later they develop a sudden fever, headache, body aches, unusual tiredness, or sometimes a rash, and some also feel nauseated or notice swollen lymph nodes. The tricky part is that most people infected with West Nile virus feel nothing at all, so when symptoms do show up, they often resemble a mild flu-like illness rather than something more specific. In a small number of people, especially older adults or those with weakened immune systems, the first signs of West Nile fever can quickly become more serious, with severe headache, neck stiffness, confusion, weakness, or trouble walking, which is a reason to seek urgent medical care.
Types of West nile fever
With West Nile fever, the day-to-day impact can range from nothing noticeable at all to a sudden illness with fever, body aches, or, in a small number of people, serious brain or nerve problems. Symptoms don’t always look the same for everyone. When doctors talk about the types of West Nile fever, they usually mean these different clinical forms of infection rather than separate strains you can feel on your own. Age, immune health, and other medical conditions can affect how severe symptoms become.
No symptoms
Most people infected with West Nile virus do not feel sick and may never know they had the infection. This form does not cause noticeable day-to-day symptoms, but the body still clears the virus.
Mild febrile illness
Some people develop the more typical form of West Nile fever, with fever, headache, tiredness, body aches, nausea, or a skin rash. One person might mostly feel wiped out for a few days, while another notices more fever and muscle pain.
Neuroinvasive disease
A small number of people develop a severe form that affects the brain, the tissues around the brain, or the spinal cord. Symptoms can include high fever, severe headache, confusion, neck stiffness, weakness, tremor, trouble walking, or paralysis.
Can I slow down the onset of symptoms?
No, there is no proven way to slow or delay the start of West Nile fever symptoms once infection has happened. What can help is lowering the chance of getting infected in the first place by avoiding mosquito bites with repellent, long sleeves, window screens, and by emptying standing water around the home; these steps reduce risk, but they do not guarantee prevention. There is no approved medicine or routine human vaccine that can reliably stop West Nile fever after exposure, but practical mosquito-avoidance habits can still make a real difference.
What genetics can tell you about symptoms
Genetics can explain a small part of why West Nile fever affects people differently, but it usually does not predict symptoms with much certainty. Studies suggest that natural differences in immune-system genes may influence whether someone has no symptoms, a mild flu-like illness, or, more rarely, serious brain or nerve problems, and they may also affect how severe the illness becomes. Still, age, overall health, and immune status are usually more important than genetics in shaping when symptoms start and how sick someone gets.
Causes and risk factors
West Nile fever usually starts after a bite from a mosquito carrying West Nile virus. Risk is higher in warm months and in places with standing water, where mosquitoes breed more easily. Spending time outside at dawn or dusk can raise exposure, and so can skipping repellent or long sleeves. Older adults and people with weak immune systems or certain long-term health problems are more likely to get severe West Nile fever. On top of genetics, everyday habits also matter, and early research suggests that small genetic differences in immune response may affect how sick some people become.
What are the most common causes or risk factors?
Most people get West Nile fever after a bite from an infected mosquito, especially in warmer months or in places with standing water where mosquitoes breed. Risk is higher for people who spend a lot of time outdoors, especially around dawn and dusk, or who live in areas where the virus is spreading among birds and mosquitoes. Older adults and people with weakened immune systems are more likely to become seriously ill, and there are no well-established inherited genetic risk factors for most people.
Environmental and biological risk factors
West Nile fever becomes more likely when infected mosquitoes are active and people are in places where the virus is circulating. Doctors often group risks into internal (biological) and external (environmental). Some factors mainly raise the chance of being bitten by an infected mosquito, while others make serious illness more likely after infection.
West Nile fever usually starts with the bite of an infected mosquito. The more contact there is with infected mosquitoes in an area, the higher the risk of infection.
Risk rises in late spring, summer, and early fall, when mosquitoes are most active. Warm weather can also help the virus multiply inside mosquitoes, which can increase spread.
Pools of still water, such as clogged gutters, buckets, birdbaths, or poorly drained areas, give mosquitoes places to breed. More breeding sites nearby can mean more mosquitoes and a greater chance of West Nile fever.
Risk is higher in regions where the virus is already circulating in mosquitoes and birds. This is why public health warnings about local West Nile fever activity matter.
Genetic risk factors
West Nile fever is not something people inherit directly from a parent. Instead, inherited differences in immune-system genes may affect how well the body controls the virus and whether illness stays mild or becomes more serious. The genetic risk factors for West Nile fever are still being studied, and most seem to change risk rather than determine the outcome. Risk is not destiny—it varies widely between individuals.
Interferon genes: Genes that control interferon, one of the body’s earliest antiviral signals, may influence how well West Nile virus is held in check. Rare inherited problems in this pathway can increase the risk of severe infection, but they do not explain most West Nile fever cases.
OAS1 changes: Inherited changes in OAS1, a gene that helps cells block viruses, have been linked to differences in the body’s response to West Nile virus. Some versions appear to raise the chance of symptomatic or more severe West Nile fever.
CCR5 deletion: A well-known inherited change called the CCR5 delta-32 deletion has been associated with a higher risk of symptoms and nerve-system complications in some people. CCR5 helps guide immune cells to where they are needed, so certain versions may weaken that defense.
HLA types: HLA genes help the immune system recognize infected cells, and inherited differences here may shape how effectively the body responds. These associations are complex, and no single HLA type can reliably predict who will become seriously ill.
Lifestyle risk factors
For West Nile fever, lifestyle mainly affects how often a person is exposed to mosquito bites rather than changing the virus itself. In practice, how lifestyle affects West Nile fever is mostly through daily habits that raise or lower contact with mosquitoes and through general health behaviors that can influence recovery. Diet and exercise are not established direct risk factors for catching West Nile fever, so the most relevant factors are exposure-related habits and preventive routines.
Spending long periods outdoors at dawn, dusk, or night can increase the chance of mosquito bites that may transmit West Nile virus. Limiting unprotected outdoor activity during peak mosquito hours can lower risk.
Skipping insect repellent on exposed skin makes bites more likely during mosquito season. Using an effective repellent consistently is one of the most important lifestyle risk factors for West Nile fever that people can modify.
Wearing shorts, sleeveless tops, or other clothing that leaves more skin uncovered can increase bite exposure. Long sleeves, long pants, and light-colored clothing can help reduce contact with mosquitoes.
Leaving windows unscreened, doors open, or sleeping without basic mosquito barriers can increase indoor biting risk. Routine use of intact screens, nets when needed, and indoor bite prevention lowers exposure.
Risk prevention
West Nile fever is mostly prevented by avoiding mosquito bites and limiting places where mosquitoes breed. For many, this means thinking ahead before an evening walk, gardening at dusk, or sitting outside near standing water. Prevention can mean both medical steps, like vaccines, and lifestyle steps, like exercise.
Use an EPA-registered insect repellent on exposed skin when mosquitoes are active, especially from dusk to dawn. This is one of the most effective ways to lower the risk of West Nile fever.
Wear long sleeves, long pants, socks, and closed shoes when spending time outdoors in mosquito season. Light-colored, loose-fitting clothes can make bites less likely and may be easier to tolerate in hot weather.
Keep window and door screens in good repair so mosquitoes stay outside. If screens are damaged, even a small tear can let mosquitoes in during the evening or overnight.
Empty or refresh water in buckets, flowerpot saucers, birdbaths, pet bowls, kiddie pools, and gutters at least weekly. Mosquitoes can breed in very small amounts of still water around the home.
Can you prevent West nile fever?
You cannot prevent West Nile fever completely, but you can lower your risk a lot because it usually spreads through mosquito bites. The best-known steps are to avoid mosquito bites by using insect repellent, wearing long sleeves and pants, and keeping window screens in good repair, especially around dawn and dusk when mosquitoes are often most active. It also helps to empty standing water around your home, such as in buckets, flowerpots, or birdbaths, because that is where mosquitoes breed.
Transmission
If you are wondering how West Nile fever spreads, it is usually passed on through the bite of an infected mosquito, most often after that mosquito has fed on infected birds. For many, this means the risk is tied to mosquito exposure outdoors, especially in warmer months, not to being around someone who is sick.
West Nile fever is not considered contagious in everyday life, so it does not spread through coughing, sneezing, kissing, touching, or sharing food. Rarely, the virus can be passed through blood transfusion, organ transplant, during pregnancy, or through breast milk, but this is uncommon and is closely screened for in medical settings.
When to test your genes
For West Nile fever, genetic testing is not used for early detection or to tailor routine care. If you have symptoms after mosquito exposure, the right tests are for the virus itself, usually blood tests and sometimes spinal fluid tests, not your genes.
Diagnosis
West Nile fever is usually diagnosed by putting together your symptoms, the time of year, and possible mosquito exposure, then confirming the cause with lab tests. Many people with West Nile fever have a mild illness, but some develop severe symptoms affecting the brain or nervous system, so the diagnosis can vary by how sick someone is. Doctors usually begin with a medical history and exam, and the diagnosis of West Nile fever is often confirmed with blood tests.
Your doctor will ask about fever, headache, body aches, rash, and whether you have been around mosquitoes or spent time outdoors. Travel, local outbreaks, and the season can also help show whether West Nile fever is a likely cause.
The exam checks for signs of dehydration, rash, and other clues of infection. If there is confusion, weakness, severe headache, or neck stiffness, doctors also look for signs that the brain or spinal cord may be involved.
Blood tests are the main way to confirm West Nile fever. They usually look for antibodies called IgM, which often appear soon after infection, while other lab tests may help rule out common conditions.
If someone has severe headache, stiff neck, seizures, confusion, or other neurologic symptoms, a spinal tap may be needed. Testing the fluid around the brain and spinal cord can support the diagnosis and help check for meningitis or encephalitis.
How is West nile fever diagnosed?
West Nile fever is usually diagnosed based on your symptoms, a doctor’s exam, and whether you may have been bitten by mosquitoes in an area where the virus is active. To confirm it, doctors most often use a blood test to look for signs that your immune system is reacting to the virus, and sometimes they also look directly for the virus itself. If someone has severe symptoms such as confusion, a very bad headache, or signs of brain or nerve problems, doctors may also test the fluid around the brain and spinal cord.
Stages of West nile fever
West Nile fever does not have defined progression stages. That is because it usually does not move through a clear step-by-step pattern over time; many people have no symptoms, some develop a short flu-like illness, and a smaller number become seriously unwell more suddenly. Doctors usually start with a conversation about symptoms such as fever, headache, body aches, rash, or the early symptoms of West Nile fever, along with possible mosquito exposure and travel. If West Nile fever is suspected, diagnosis is usually based on an exam and lab tests, most often blood tests and sometimes spinal fluid tests if there are signs the brain or nerves may be affected.
Outlook and prognosis
Most people with West Nile fever recover fully, often within days to weeks, although fatigue, weakness, or difficulty concentrating can sometimes last for weeks or months. The outlook is generally excellent for uncomplicated infection, because many cases are mild or even asymptomatic. Prognosis is more serious when the infection becomes neuroinvasive, causing meningitis, encephalitis, or acute flaccid paralysis, which can lead to prolonged disability and slower, sometimes incomplete, recovery. Older adults, people with weakened immune systems, and those with certain chronic medical conditions are at higher risk of severe illness and death. Overall mortality is low in typical West Nile fever, but among patients with neuroinvasive disease, death can occur, with reported fatality rates varying by age and severity.
Long-term effects
Most people with West Nile fever recover fully, especially if the illness was mild. Still, some living with West Nile fever have health problems that last for months or, less often, years, and these are more common after infection of the brain or nervous system. Long-term effects vary widely, from lingering tiredness to problems with movement, memory, or daily independence. Older adults and people who had severe neurologic illness usually have a higher chance of lasting effects.
Many people with West Nile fever feel unusually tired long after the fever has passed. For some, even ordinary tasks like shopping, working, or climbing stairs can take more effort than before.
Weakness can persist, especially after severe West Nile fever that affected the nerves or spinal cord. Some may notice trouble lifting a foot, gripping objects, or standing from a chair without support.
Some people have ongoing trouble with attention, short-term memory, or mental speed after West Nile fever. This can show up as forgetting appointments, losing track in conversations, or feeling mentally slowed at work.
Anxiety, low mood, irritability, and sleep problems can continue during recovery. Some effects may be emotional, while others can be tied to lasting changes in the brain and the stress of a serious illness.
How is it to live with West nile fever?
Living with West Nile fever is often a short, draining stretch of illness marked by fever, body aches, headache, and deep fatigue that can make work, exercise, and even routine chores feel much harder than usual. For many people, rest and hydration are the main focus for days to weeks, and even after the fever fades, low energy can linger for a while before normal life fully returns. Around you, family and friends may need to help with daily tasks and provide reassurance, but casual contact does not spread West Nile virus, so the bigger shared concern is usually mosquito prevention rather than person-to-person transmission.
Treatment and drugs
Treatment for West Nile fever is mainly supportive, because there is no specific antiviral drug that reliably cures the infection. Supportive care can make a real difference in how you feel day to day, and for many people this means rest, fluids, and medicine to lower fever or ease headache and body aches. Most people with West Nile fever recover at home, but those who develop more serious illness may need hospital care for intravenous fluids, breathing support, pain control, or close monitoring of brain and nerve problems. In severe cases, treatment focuses on managing complications rather than removing the virus itself. If early symptoms of West Nile fever are getting worse, or if confusion, weakness, trouble walking, or a stiff neck appear, urgent medical care is important.
Can you cure West nile fever?
No, there is no specific cure for West Nile fever right now. Treatment focuses on helping the body recover and easing symptoms with rest, fluids, and medicine for pain or fever, while people who get very sick may need hospital care such as IV fluids or help with breathing.
Non-drug treatment
West Nile fever is usually managed with supportive care rather than a specific cure. For many, treatment begins not at the pharmacy, but with rest, fluids, and close attention to symptoms while the body clears the infection. Most people with West Nile fever recover at home, but those with severe illness, especially brain or nerve involvement, may need hospital-based therapies. If you are worried about early symptoms of West Nile fever, worsening weakness, confusion, or trouble breathing should be checked urgently.
Extra rest helps your body recover while the infection runs its course. Many living with West Nile fever find it helps to scale back work, exercise, and other demanding activities until strength returns.
Drinking enough water, broth, or oral rehydration drinks can help if fever, sweating, or poor appetite leave you drained. Small, simple meals may be easier to manage when nausea or fatigue makes normal eating hard.
Light clothing, a cool room, and lukewarm compresses can make fever and body aches easier to tolerate. These steps do not treat the virus itself, but they can improve comfort, especially during the first few days.
Severe West Nile fever may require observation in the hospital to watch breathing, hydration, and changes in alertness. Non-drug treatments may be recommended when there is concern for meningitis, encephalitis, or weakness affecting the nerves.
Your genes may have an impact on how drugs work for you
For West Nile fever, treatment is usually about easing symptoms, such as rest, fluids, and medicines for fever or pain, rather than using a specific drug that targets the virus. In general, small differences people are born with can change how fast the body breaks down some medicines or how likely side effects are, so one person may feel better with a usual dose while another may be more sensitive. That said, for West Nile fever this is not usually a major part of care, and genetic testing is not a routine step because treatment is mainly supportive.
Pharmacological treatments
Most people with West Nile fever do not need a specific antiviral drug, because there is no approved medicine that directly clears the virus. Treatment usually focuses on easing fever, body aches, headache, nausea, and other day-to-day symptoms while the body recovers. Drugs that target symptoms directly are called symptomatic treatments, and this is the main approach to treatment for West Nile fever symptoms.
Acetaminophen is often used to lower fever and ease headache or body aches in West Nile fever. It can be a good option when pain and fever are making it hard to rest or drink enough fluids.
Ibuprofen or naproxen may also help with fever, muscle pain, and joint pain. These medicines are not right for everyone, especially if there is stomach bleeding risk, kidney disease, dehydration, or certain other health conditions.
Medicines such as ondansetron may be used if nausea or vomiting makes it hard to keep fluids down. For many, this can mean fewer trips to the bathroom and a better chance of avoiding dehydration.
In severe West Nile fever that affects the brain or nervous system, anti-seizure drugs such as levetiracetam or phenytoin may be needed if seizures occur. These are usually given in the hospital and are aimed at controlling complications rather than treating the virus itself.
Genetic influences
West Nile fever is not a condition people inherit directly from a parent; it is an infection most often picked up from the bite of an infected mosquito. Your genes may still play a small part in how your immune system responds once you are exposed, which may help explain why some people have a mild illness while others are more likely to develop serious problems affecting the brain or nerves. Researchers have found that natural differences in genes involved in the body’s antiviral defenses can influence risk, but these effects are usually modest and are not something doctors use in routine care.
For many living with West Nile fever, the bigger drivers of severe illness are age, immune suppression, and other health conditions rather than a single inherited cause. This means genetics may help answer why some people get severe West Nile fever, but family history is not usually the main clue the way it is for an inherited disorder. At this point, genetic testing is not a standard part of diagnosing or managing West Nile fever.
Is West nile fever genetic?
Genetics may slightly influence how West Nile fever affects someone, including the chance of severe illness, but this is not the main driver. For most people, mosquito exposure, older age, and immune health matter much more than genetic variations.
Having a genetic risk is not necessarily a diagnosis
Genetic variants are a usual part of human biology and are widely found in the general population. Most of these variants do not directly affect health and simply reflect natural human diversity. For many genes, having a variant connected to a condition suggests a possible link rather than a certain diagnosis, especially when more than one gene and environmental factors are involved. In some cases, though, a single well-understood variant in a specific gene is enough to confirm a diagnosis.
Pharmacogenetics—how genetics influence drug effects
For most people with West Nile fever, treatment focuses on relieving symptoms like fever, headache, body aches, and dehydration, rather than using a virus-specific drug. Because there is no standard antiviral medicine approved specifically for West Nile fever, genetic testing does not usually guide routine treatment the way it can for some cancers or heart conditions. Genetics is only one factor, and with West Nile fever, age, immune health, pregnancy, and how severe the infection becomes usually matter more when doctors choose care. Even so, inherited differences can affect how your body handles some medicines used during supportive care, such as pain relievers, anti-nausea drugs, or medicines given in the hospital for seizures or other complications. This can help explain part of how genetics affect treatment for West Nile fever, although it is not yet a standard part of care for most people living with West Nile fever. Researchers are also studying whether certain immune-system genes are linked to more severe illness, but these findings are not yet used to routinely select drugs for West Nile fever.
Interactions with other conditions
West Nile fever can be harder on people who already have health conditions that affect the immune system, such as cancer treatment, an organ transplant, or certain long-term inflammatory illnesses. Older adults and people living with diabetes, kidney disease, or high blood pressure also tend to have a higher risk of severe West Nile fever, including infection of the brain or the tissues around it. Day-to-day, it can feel like one health issue amplifies the other, because fever, weakness, dehydration, and confusion may worsen problems a person is already managing. In some cases, early symptoms of West Nile fever can also look like other infections or flare-ups of existing neurologic conditions, which can delay recognition. Some may find that recovery takes longer if they were already frail or had trouble with balance, memory, or muscle strength before getting sick. If you have other medical conditions, it is important to let your doctor know, since they may want to watch more closely for complications and check whether any symptoms are coming from West Nile fever, another illness, or both.
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 West nile fever?
West Nile fever can look a lot like other common infections, especially the flu, COVID-19, and other virus illnesses that cause fever, headache, body aches, tiredness, or sometimes a rash. In areas where mosquito-borne illnesses are a concern, doctors may also consider dengue, Zika, or malaria, and if there is a stiff neck, confusion, or a very bad headache, other infections that affect the brain or the tissues around it can cause similar symptoms too.
History
For many people, West Nile fever first comes up in a very practical way: hearing about mosquitoes in late summer, local public health alerts, or a neighbor who suddenly develops fever, body aches, and fatigue after spending time outdoors. The illness takes its name from the West Nile district of Uganda, where it was first identified in 1937 in a woman with a fever during research on infectious diseases.
Over time, doctors learned that West Nile fever was not limited to one region. Cases were later recognized in parts of Africa, the Middle East, Europe, and western Asia, and by the mid-to-late 20th century it had become clear that outbreaks could sometimes involve the brain and nervous system, not just a short flu-like illness. In recent decades, awareness has grown as health systems improved testing and tracking, helping doctors distinguish West Nile fever from other causes of summer fever, rash, meningitis, and encephalitis.
A major turning point came in 1999, when West Nile virus was detected in New York City during an outbreak linked to severe neurologic illness and deaths in both people and birds. That event showed how quickly mosquito-borne infections can appear in new places and led to much wider mosquito surveillance, blood donor screening, and public health monitoring in the United States. Since then, West Nile fever has been reported seasonally across much of North America, with patterns that shift from year to year depending on mosquito activity, weather, bird populations, and local conditions.
The history of West Nile fever also changed how doctors think about its symptoms. Early on, many infections were probably missed because some people have no symptoms at all, while others develop only mild illness. As testing improved, it became easier to recognize the early symptoms of West Nile fever and to understand that a smaller number of people, especially older adults and those with certain health conditions, can become seriously ill. That fuller picture has helped shape today’s advice on mosquito prevention, outbreak response, and when to seek medical care.