Mosquitoes carry more than a dozen infectious diseases, making them the deadliest animals on the planet. The most significant are malaria, dengue, Zika, chikungunya, West Nile virus, yellow fever, and several types of encephalitis. Together, these diseases cause hundreds of millions of infections and more than 600,000 deaths every year.
Malaria
Malaria is the single deadliest mosquito-borne disease. It caused an estimated 282 million cases and 610,000 deaths in 2024, with about two thirds of global cases concentrated in just 11 countries in sub-Saharan Africa. The disease is caused by a parasite, not a virus, which makes it unique among most mosquito-transmitted illnesses. Anopheles mosquitoes carry the parasite and inject it when they bite.
Symptoms typically appear 10 to 15 days after a bite and include high fever, chills, sweating, headache, and body aches. Without treatment, malaria can progress to organ failure and death within days, particularly in young children. Effective treatment exists, and travelers to endemic areas can take preventive medication. Malaria was eliminated from the United States decades ago, but Anopheles mosquitoes still live throughout the continental U.S., which is why occasional locally transmitted cases still pop up.
Dengue
Dengue is the most widespread mosquito-borne viral disease. More than 3.9 billion people in over 132 countries live in areas where they could catch it, with roughly 96 million symptomatic cases and 40,000 deaths each year. The virus is spread primarily by the Aedes aegypti mosquito.
Symptoms begin 4 to 10 days after a bite and typically last 2 to 7 days. The classic signs are high fever (often hitting 104°F), severe headache, pain behind the eyes, muscle and joint pain, nausea, swollen glands, and rash. Most people recover on their own, but a small percentage develop severe dengue, which involves internal bleeding, plasma leakage, and can be fatal without hospital care. A second dengue infection with a different strain of the virus carries a higher risk of severe disease than the first.
In the United States, dengue outbreaks have occurred in Florida, Hawaii, Texas, Puerto Rico, the U.S. Virgin Islands, and American Samoa.
Zika Virus
Zika made global headlines during the 2015-2016 outbreak because of its devastating effects on pregnancy. The virus can cause microcephaly and other serious brain defects in developing fetuses. For most adults, Zika itself is mild. The most common symptoms are rash (nearly always present), low-grade fever, pink eye, and joint pain. Many people never realize they’re infected.
Zika is spread by the same Aedes mosquitoes that carry dengue and can also be transmitted sexually, which is unusual for a mosquito-borne virus. Large-scale outbreaks have subsided since 2017, but the virus still circulates in tropical regions, and the mosquitoes that carry it continue to expand their range.
Chikungunya
Chikungunya is often confused with dengue because both are carried by Aedes mosquitoes and share overlapping symptoms. The key difference is joint pain. Chikungunya causes intense, debilitating joint pain that is nearly always present and can persist for months or even years after the initial infection. Fever is also high and almost universal. Rash is common but less prominent than with Zika.
The disease is rarely fatal, but the prolonged joint pain can be seriously disabling. There’s no specific antiviral treatment, so management focuses on rest, fluids, and pain relief. A vaccine was approved in the U.S. in late 2023 for adults traveling to areas where the virus circulates.
West Nile Virus
West Nile is the most common mosquito-borne disease in the continental United States. It’s spread by Culex mosquitoes, a different group than the Aedes species responsible for dengue and Zika. In 2025, 47 states reported human cases, totaling 2,076 infections. Colorado (285 cases), Illinois (149), Texas (127), Minnesota (122), and California (112) had the highest numbers.
About 80% of people infected with West Nile never develop symptoms. Around 20% get a flu-like illness with fever, headache, and body aches. Fewer than 1% develop neuroinvasive disease, where the virus infects the brain or spinal cord, causing encephalitis or meningitis. But that small percentage still accounted for 1,434 serious cases in 2025. Older adults and people with weakened immune systems face the highest risk of severe outcomes. There is no vaccine or specific treatment for West Nile.
Yellow Fever
Yellow fever is a viral disease that still kills an estimated 30,000 to 60,000 people per year, mostly in sub-Saharan Africa and tropical South America. It gets its name from the jaundice (yellowing of the skin) that occurs when the virus damages the liver. Mild cases cause fever, muscle pain, and nausea. Severe cases progress to liver and kidney failure, bleeding, and death, with a fatality rate of roughly 20 to 50% among those who reach the severe phase.
Unlike most mosquito-borne diseases, yellow fever has a highly effective vaccine that provides lifelong protection with a single dose. Many countries in endemic zones require proof of vaccination for entry. The CDC recommends vaccination for all travelers nine months and older heading to areas with endemic or transitional yellow fever risk, which spans large portions of tropical Africa and South America.
Eastern Equine Encephalitis
Eastern equine encephalitis (EEE) is rare but extraordinarily dangerous. It has an estimated 30% fatality rate, and roughly half of survivors are left with permanent neurological damage, including cognitive impairment, personality changes, and paralysis. The virus circulates between birds and mosquitoes in freshwater swamps along the eastern United States, occasionally spilling over to humans.
Only a handful of human cases occur most years, but outbreaks can spike unpredictably. Symptoms of severe infection include sudden high fever, headache, vomiting, and rapidly progressing confusion or seizures. There is no vaccine for humans and no antiviral treatment. The rarity of the disease makes it easy to overlook, but the outcomes when infection does occur are among the worst of any mosquito-borne illness.
Which Mosquitoes Carry Which Diseases
Not all mosquitoes transmit the same diseases. The three groups responsible for most human illness are distinct in their habits and the pathogens they carry:
- Aedes (especially Ae. aegypti and Ae. albopictus): Dengue, Zika, chikungunya, and yellow fever. These mosquitoes bite during the day, prefer urban environments, and breed in small containers of standing water like flower pots, tires, and buckets.
- Anopheles: Malaria. These mosquitoes tend to bite between dusk and dawn and breed in clean, unpolluted water sources.
- Culex: West Nile virus and several types of encephalitis. They bite mostly at dusk and dawn, breed in stagnant water, and are the most common mosquitoes across much of the U.S.
This distinction matters for prevention. If you’re in an area with dengue risk, daytime protection is essential because Aedes mosquitoes are aggressive daytime biters. If West Nile is your concern, focus on dusk and dawn, when Culex mosquitoes are most active.
Why Mosquito-Borne Diseases Are Spreading
Rising temperatures and shifting rainfall patterns are pushing Aedes mosquitoes into regions where they couldn’t previously survive. Both Ae. aegypti and Ae. albopictus (the Asian tiger mosquito) have expanded significantly into temperate regions that were once too cool to support them. This means diseases like dengue, Zika, and chikungunya are appearing in places with no prior history of local transmission, including parts of southern Europe and the southern United States.
Urbanization accelerates the problem. Aedes mosquitoes thrive in cities, where dense populations and abundant breeding sites (any small container that holds water) create ideal conditions. Global travel also plays a role: a person infected with dengue in Southeast Asia can land in a U.S. city with Aedes mosquitoes present, and local transmission can follow. The combination of expanding mosquito habitat, growing cities, and international travel means the geographic footprint of these diseases will likely continue to grow.