Mosquito-borne diseases: real risks, plainly explained

The short answer

Most mosquito bites are harmless. They itch, they annoy, and then they fade. The reason mosquitoes matter for health is not the average bite. It is a small set of diseases that a small share of mosquitoes can carry.

Two things are true at once, and holding both is the key to thinking clearly here. First, the everyday risk from any single bite is very low, and most people who do get infected with a mosquito-borne virus feel nothing at all or have a mild, flu-like illness. Second, a handful of these diseases can be serious, especially for certain groups, and a few of them can be life-changing or fatal in rare cases.

So the honest posture is neither panic nor dismissal. It is calibrated caution. Learn which diseases actually occur where you live and travel, know who in your household is at higher risk, take a few simple prevention steps, and know the point at which symptoms are worth a call to a doctor. This page is the map. Each disease has its own page with the detail.

How mosquitoes spread disease

A mosquito is not born infected. It picks up a virus or parasite by biting an infected animal or person, and after the pathogen develops inside the mosquito for a number of days, the mosquito can pass it to the next host it bites. Different diseases rely on different mosquito species and different animal reservoirs, which is why the where, the when, and the how-worried differ so much from one disease to the next. For the full mechanism, including why only certain species matter and why season and weather change the risk, see how mosquitoes spread disease.

The diseases to know here

This is a scannable overview. The “how worried to be” column is meant to be honest and calibrated, not alarming. Rare-but-severe and common-but-mild are both real, and they call for different responses. Follow any row to its full page for symptoms, warning signs, and specifics.

A few things stand out from that table. West Nile is the one most people in the continental region will actually encounter, even if they never know it. EEE is the rare exception where low odds meet very high stakes. Dengue, Zika, chikungunya, malaria, and yellow fever are, for people here, mostly diseases of travel, though dengue has shown small pockets of local transmission in a few warm states. And heartworm is a reminder that the household members most exposed to mosquito-borne disease are often the dogs.

Who is at higher risk

For most mosquito-borne viruses, the same groups tend to face higher odds of severe illness:

• Older adults. Age is the single biggest risk factor for severe West Nile disease. People 60 and older, and especially those 70 and older, are far more likely to develop the neurologic form. St. Louis encephalitis and Jamestown Canyon virus also hit older adults harder.
• Very young children. La Crosse encephalitis is the standout here; most cases occur in children, who are the most vulnerable to its severe, neurologic form.
• Pregnant people. The specific concern is Zika, which can cause serious birth defects when a person is infected during pregnancy. This is why travel guidance for pregnancy is stricter.
• People who are immunocompromised. Cancer treatment, organ transplants, and other conditions that weaken the immune system raise the risk of severe outcomes, particularly for West Nile.
• People with certain chronic conditions. Diabetes, high blood pressure, and kidney disease are associated with higher risk of severe West Nile illness.
• Pets. Dogs and cats cannot tell you they are at risk. Heartworm is preventable and treatable-if-caught, but far easier to prevent than to treat, which is why year-round prevention is the standard advice.

If someone in your household is in one of these groups, prevention is worth a little more attention, and a fever with confusion, severe headache, or stiff neck is worth a faster call to a doctor.

How to lower your risk

The good news is that the same simple habits protect against nearly every disease on this page, because they all come down to the same thing: fewer bites, and fewer mosquitoes breeding near you.

Prevent bites.
– Use an EPA-registered insect repellent. Effective active ingredients include DEET, picaridin, IR3535, oil of lemon eucalyptus (or its synthesized form PMD), and 2-undecanone. Used as directed, these are considered safe, including during pregnancy and breastfeeding.
– Wear loose, long sleeves and pants when mosquitoes are active, and consider clothing or gear treated with permethrin.
– Use screens on windows and doors, and repair holes.

Remove standing water. Mosquitoes lay eggs in still water, and some of the species that matter breed in surprisingly small amounts. Once a week, empty, scrub, cover, or throw out anything that holds water: buckets, tires, toys, plant saucers, birdbaths, trash bins, and clogged gutters.

For the full playbook, including which repellents last longest and how to find breeding sites you might be missing, see the prevention guides for bite prevention and standing water and your yard.

When to see a doctor

Most mosquito-borne infections never need medical care. But some symptoms are worth taking seriously, especially in the higher-risk groups above.

Contact a doctor if, within about two weeks of mosquito exposure or travel, you or someone you care for develops:

• A high fever, especially with a severe headache, stiff neck, confusion, disorientation, tremors, or seizures. These can signal the neurologic forms of West Nile, EEE, La Crosse, St. Louis encephalitis, or Jamestown Canyon virus, and they warrant prompt care.
• Fever after travel to an area with malaria or dengue. Malaria in particular can become serious quickly and needs urgent evaluation, so mention your travel history.
• Dengue warning signs after a fever starts to break: severe belly pain, persistent vomiting, bleeding from the gums or nose, or extreme fatigue. These call for emergency care.

There is no specific antiviral for most of these viruses, so care is supportive, which is exactly why prevention and early attention matter. When in doubt, calling a clinician is the right move.

Sources

1. CDC, “About West Nile” (leading cause of mosquito-borne disease in the continental region; roughly 8 in 10 infected have no symptoms, about 1 in 5 develop fever, about 1 in 150 develop severe neurologic illness; transmitted by Culex mosquitoes, summer into fall). https://www.cdc.gov/west-nile-virus/about/index.html Accessed 2026-07-20.
2. CDC MMWR, “West Nile Virus and Other Nationally Notifiable Arboviral Diseases, United States, 2021” (West Nile is the leading domestically acquired arboviral disease). https://www.cdc.gov/mmwr/volumes/72/wr/mm7234a1.htm Accessed 2026-07-20.
3. CDC, “Symptoms, Diagnosis, and Treatment: West Nile Virus” and related pages (higher risk in adults over 60 and people with cancer, diabetes, hypertension, kidney disease, or organ transplants; about 1 in 10 with severe neurologic illness die). https://www.cdc.gov/west-nile-virus/symptoms-diagnosis-treatment/ Accessed 2026-07-20.
4. CDC, “About Eastern Equine Encephalitis Virus” (rare; about 30% of people with severe EEE die; bridge mosquitoes transmit to mammals). https://www.cdc.gov/eastern-equine-encephalitis/about/index.html Accessed 2026-07-20.
5. Boyce RM et al., PLOS Neglected Tropical Diseases, “Geographically persistent clusters of La Crosse virus disease in the Appalachian region of the United States from 2003 to 2021.” https://journals.plos.org/plosntds/article?id=10.1371/journal.pntd.0011065 Accessed 2026-07-20.
6. UNC Institute for Global Health and Infectious Diseases, “La Crosse Encephalitis Virus” (spread by the eastern treehole mosquito, Aedes triseriatus; most cases in children; upper Midwest and Appalachia). https://globalhealth.unc.edu/2023/04/la-crosse-encephalitis-virus-understanding-a-rare-mosquito-borne-disease-that-affects-children-in-the-appalachian-region/ Accessed 2026-07-20.
7. CDC, “About St. Louis Encephalitis Virus” (Culex-transmitted; most infections mild; severe disease more likely in older adults; eastern and central states). https://www.cdc.gov/sle/about/index.html Accessed 2026-07-20.
8. CDC, “About Jamestown Canyon Virus” (many infections symptom-free; found across much of the region, most cases in the upper Midwest; late spring to mid-fall; severe disease more likely in older or immunocompromised people). https://www.cdc.gov/jamestown-canyon/about/index.html Accessed 2026-07-20.
9. CDC MMWR, “Increase in Travel-Associated and Locally Acquired Dengue Cases, United States, 2024” (2024 cases up 359% over the 2010 to 2023 average; 97.2% travel-associated, 2.8% locally acquired; local transmission in FL, TX, HI, AZ, CA; Aedes aegypti and Aedes albopictus vectors). https://www.cdc.gov/mmwr/volumes/75/wr/mm7518a1.htm Accessed 2026-07-20.
10. CDC, “About Zika” (Aedes aegypti primary vector; infection in pregnancy can cause microcephaly and other serious birth defects; local mosquito-borne transmission last reported in FL and TX in 2016). https://www.cdc.gov/zika/about/index.html Accessed 2026-07-20.
11. CDC Yellow Book, “Chikungunya” and CDC “Symptoms of Chikungunya” (Aedes aegypti and albopictus; fever and often severe joint pain that can persist; more than 4,000 cases among US travelers and 13 locally acquired cases in the continental region after the 2014 to 2017 outbreaks). https://www.cdc.gov/chikungunya/signs-symptoms/index.html Accessed 2026-07-20.
12. CDC MMWR, “Outbreak of Locally Acquired Mosquito-Transmitted (Autochthonous) Malaria, Florida and Texas, May to July 2023” (first locally acquired US cases in 20 years; Anopheles vectors). https://www.cdc.gov/mmwr/volumes/72/wr/mm7236a1.htm Accessed 2026-07-20.
13. CDC, “Data and Statistics on Malaria in the United States” (nearly all US cases are travel-associated; record imported cases in 2023). https://www.cdc.gov/malaria/php/surveillance-report/index.html Accessed 2026-07-20.
14. CDC Yellow Book, “Yellow Fever” (Aedes aegypti in the urban cycle; last US transmission during the 1905 New Orleans epidemic; a travel risk in parts of sub-Saharan Africa and South America; vaccine-preventable). https://www.cdc.gov/yellow-book/hcp/travel-associated-infections-diseases/yellow-fever.html Accessed 2026-07-20.
15. American Heartworm Society, “Heartworm Guidelines,” and Companion Animal Parasite Council, “Heartworm” (Dirofilaria immitis spread by mosquitoes including Aedes, Culex, Anopheles, and Mansonia; diagnosed in all 50 states; year-round prevention recommended). https://www.heartwormsociety.org/veterinary-resources/american-heartworm-society-guidelines and https://capcvet.org/guidelines/heartworm/ Accessed 2026-07-20.
16. CDC, “Preventing Mosquito Bites” (EPA-registered repellents with DEET, picaridin, IR3535, oil of lemon eucalyptus / PMD, or 2-undecanone; long sleeves and permethrin-treated clothing; empty and scrub standing water weekly). https://www.cdc.gov/mosquitoes/prevention/index.html Accessed 2026-07-20.

Explore the diseases

• West Nile virus
• Eastern equine encephalitis (EEE)
• La Crosse encephalitis
• St. Louis encephalitis
• Jamestown Canyon virus
• Dengue
• Zika virus
• Chikungunya
• Malaria (travel)
• Yellow fever
• Dog heartworm
• How mosquitoes spread disease

This page is a general reference and is not medical advice. If you are worried about symptoms in yourself, a family member, or a pet, contact a licensed clinician or veterinarian. Last reviewed 20 July 2026.