Who is most at risk from mosquito diseases

The short answer

Most people who get bitten by an infected mosquito are fine. For the main mosquito-borne viruses found here, the large majority of infections cause either no symptoms at all or a mild, flu-like illness that passes on its own.

A smaller number of people face higher odds of serious illness. The single most important factor is age, followed by pregnancy, a weakened immune system, and certain chronic health conditions. If you are in one of these groups, the sensible response is not alarm. It is steady, consistent attention to preventing mosquito bites in the first place.

To put the numbers in perspective: with West Nile virus, the most common of these diseases, about 8 in 10 people who are infected develop no symptoms, and roughly 1 in 150 develops a severe illness that affects the brain or spinal cord (CDC). Higher risk shifts those odds. It does not guarantee a bad outcome.

Why age is the biggest factor

For most mosquito-borne viruses in the country, age is the strongest predictor of how serious an infection is likely to be.

Older adults carry the highest risk of severe disease. With West Nile virus, people aged 60 and older, along with people who have certain medical conditions, are at greater risk for serious illness (CDC). The pattern grows steeper with each decade: adults aged 60 to 69 are about twice as likely, and those aged 70 and older more than six times as likely, to be hospitalized with West Nile disease compared with younger adults. Among people who develop the severe, brain-involving form of the disease, deaths are most common in those aged 70 and older, at roughly 20 percent, compared with about 2 percent for people under 50 (CDC).

Age cuts both ways for some viruses. A few affect the very young most heavily. La Crosse encephalitis, spread by mosquitoes in the upper Midwest, mid-Atlantic, and southeastern states, most often causes severe disease in children under 16 (CDC). Eastern equine encephalitis (EEE), a rare but very serious infection, appears to hit hardest at both ends of the age range: people over 50 and children under 15 seem to be at greatest risk of severe disease (CDC).

The takeaway is not that any single age is unsafe. It is that risk is not evenly spread, and knowing where you or a family member fall helps you decide how carefully to focus on prevention.

The higher-risk groups

If you belong to one or more of the groups below, you have higher odds of a serious outcome if you are infected. That is a reason to prioritize bite prevention, not a reason to expect illness.

• Adults 60 and older. The clearest single risk factor for severe West Nile virus disease, with risk rising sharply after age 70 (CDC).
• Pregnant people. Zika virus infection during pregnancy can cause microcephaly and other serious birth defects of the developing brain (CDC). Not every infected pregnancy results in harm, but the risk is real and the reason pregnant people are urged to take bite prevention seriously and to avoid areas with active Zika spread.
• Infants and young children. For certain viruses, notably La Crosse encephalitis and EEE, young children are among the most vulnerable to severe disease (CDC).
• People who are immunocompromised, including organ-transplant recipients. People with weakened immune systems, including transplant recipients and those on immune-suppressing medications, are at higher risk of the severe, brain-involving form of West Nile disease. Neuroinvasive West Nile disease occurs at higher rates in solid-organ transplant recipients than in the general population (Kasule et al., Transplant Infectious Disease, 2023).
• People with certain chronic conditions. Cancer, diabetes, high blood pressure, and kidney disease are each linked to greater risk of severe West Nile virus disease (CDC).
• People with heavy outdoor exposure. Risk also depends on how much you are bitten. Anyone who spends long hours outdoors in mosquito season, for work or recreation, has more chances to be exposed, which matters most when combined with any of the factors above.

Higher risk does not mean high risk

It is easy to read a list like the one above and feel like a serious infection is likely. For almost everyone, it is not.

Even in the higher-risk groups, most infections are still mild or produce no symptoms at all. The severe forms of these diseases are uncommon. West Nile virus, again the most widespread of them, causes no symptoms in about 80 percent of infected people, and the severe neuroinvasive form in roughly 1 in 150 (CDC). EEE and La Crosse are serious when they occur but are rare overall, with only a small number of cases reported nationally each year.

“Higher risk” is a comparison, not a prediction. It means your odds are worse than someone outside the group, and that the payoff from preventing bites is larger for you. It does not mean a bad outcome is expected.

What to do if you are in a higher-risk group

The most useful step is also the simplest: reduce your mosquito bites. Because there is no vaccine for the mosquito-borne viruses found here, and no specific antiviral treatment for most of them, prevention does the heavy lifting.

• Use an EPA-registered insect repellent (with an active ingredient such as DEET, picaridin, IR3535, oil of lemon eucalyptus, para-menthane-diol, or 2-undecanone) and follow the label. When used as directed, these are considered safe and effective, including for pregnant and breastfeeding people (CDC; EPA).
• Wear long sleeves and long pants when mosquitoes are active, especially at dawn and dusk for the species that carry West Nile and EEE.
• Keep mosquitoes out of your home with screens on windows and doors, and use air conditioning where available.
• Empty standing water from containers around your home once a week (buckets, planters, birdbaths, gutters), since that is where mosquitoes breed.
• If you are pregnant, talk with your healthcare provider about travel, and avoid areas with active Zika transmission (CDC).

For a fuller walkthrough of repellents, clothing, and yard steps, see the prevention guides linked below.

When to see a doctor

Most mosquito bites need no medical attention. But if you develop symptoms after being bitten during mosquito season, especially if you are in a higher-risk group, do not wait to get checked out.

Seek medical care promptly if you have:

• High fever, severe headache, or a stiff neck
• Confusion, disorientation, or trouble concentrating
• Muscle weakness, tremors, or seizures
• Sudden difficulty with balance, vision, or speech

These can be signs of the brain-involving forms of West Nile, EEE, or La Crosse infection, which need urgent evaluation. If you are pregnant and think you may have been exposed to Zika, or you have traveled to an area with active spread, contact your provider even if you feel well (CDC).

When in doubt, call your doctor and mention recent mosquito exposure. It helps them decide what to test for.

Sources

1. CDC, “Symptoms of West Nile.” Roughly 8 in 10 infected people have no symptoms; about 1 in 150 develops severe (neuroinvasive) disease; adults 60 and older and people with certain conditions are at increased risk. https://www.cdc.gov/west-nile-virus/signs-symptoms/index.html (accessed 20 July 2026)
2. CDC, “Key Messages: West Nile Virus.” People over 60 and those with cancer, diabetes, hypertension, kidney disease, or an organ transplant are at greater risk for severe disease; hospitalization and death rates by age group. https://www.cdc.gov/west-nile-virus/php/outbreak-communication/key-messages.html (accessed 20 July 2026)
3. CDC, “About La Crosse.” La Crosse encephalitis most often causes severe disease in children under 16; geographic distribution. https://www.cdc.gov/la-crosse-encephalitis/about/index.html (accessed 20 July 2026)
4. CDC, “Guidelines for Eastern Equine Encephalitis Virus Surveillance and Control.” People over 50 and children under 15 appear to be at greatest risk of severe EEE disease. https://www.cdc.gov/eastern-equine-encephalitis/php/surveillance-and-control-guidelines/index.html (accessed 20 July 2026)
5. CDC, “About Eastern Equine Encephalitis Virus.” Severity and rarity of EEE. https://www.cdc.gov/eastern-equine-encephalitis/about/index.html (accessed 20 July 2026)
6. CDC (archived), “CDC Concludes Zika Causes Microcephaly and Other Birth Defects” (2016). Zika infection during pregnancy can cause microcephaly and other serious fetal brain defects. https://archive.cdc.gov/www_cdc_gov/media/releases/2016/s0413-zika-microcephaly.html (accessed 20 July 2026)
7. Kasule SN et al., “Neuroinvasive West Nile virus infection in solid organ transplant recipients,” Transplant Infectious Disease, 2023. Neuroinvasive West Nile disease occurs at higher rates in solid-organ transplant recipients than in the general population. https://onlinelibrary.wiley.com/doi/abs/10.1111/tid.14004 (accessed 20 July 2026)
8. CDC, “Preventing Mosquito Bites.” Use of EPA-registered repellents, protective clothing, screens, and removing standing water. https://www.cdc.gov/mosquitoes/prevention/index.html (accessed 20 July 2026)
9. EPA, “Repellents: Protection against Mosquitoes, Ticks and Other Arthropods.” EPA-registered repellent active ingredients; safe and effective when used as directed, including for pregnant and breastfeeding people. https://www.epa.gov/insect-repellents (accessed 20 July 2026)

Related

• Symptoms of West Nile virus and when they turn serious [placeholder]
• How to choose and use an insect repellent that works [placeholder]
• Mosquito-borne diseases in the country: an overview [placeholder]
• La Crosse encephalitis and children: what parents should know [placeholder]
• Zika and pregnancy: a practical guide [placeholder]
• Reducing mosquitoes around your home [placeholder]

This page is for general information and does not replace advice from a qualified healthcare professional. If you are concerned about symptoms or your individual risk, contact your doctor or local health department.

Last reviewed 20 July 2026.