Mosquitoes are more than just a summertime nuisance—they are carriers of viruses and parasites that can lead to serious illnesses in humans. In the United States, diseases like West Nile virus are among the most common mosquito-borne threats, though most infections tend to be mild. Still, as climate change contributes to rising global temperatures and shifting ecosystems, mosquito populations are expanding, increasing the risk of disease transmission.
“Most mosquito bites are not going to result in West Nile virus because the vast majority of mosquitoes are not infected,” explains Dr. Marc Siegel, professor of medicine at the George Washington School of Medicine and Health Sciences. Nevertheless, the risk remains real, and prevention is key.
With over 200 species of mosquitoes found in the U.S., only about a dozen are capable of spreading viruses harmful to humans. Yet even one infected bite can have serious consequences. As mosquitoes thrive in warmer, wetter conditions, understanding how to avoid exposure is more important than ever.
From the best times to stay indoors to the most effective protective measures outdoors, this guide breaks down the top mosquito-borne illnesses, the risks they carry, and practical steps you can take to stay protected.
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Common Mosquito-Borne Illnesses and How They Impact Health
West Nile Virus
West Nile virus is the most common mosquito-borne illness in the United States, with its transmission season typically spanning from June to September. The virus naturally circulates among birds, which act as its primary hosts. When mosquitoes bite infected birds, they can then transmit the virus to humans through subsequent bites.
As of June 17, the Centers for Disease Control and Prevention (CDC) reported 16 confirmed human cases across 11 states, primarily in the Southeastern and Midwestern regions, such as Louisiana and Illinois. While infection rates remain low early in the season, public health experts caution that cases may rise later in the summer.
Most individuals infected with West Nile virus remain asymptomatic. However, about 1 in 5 people may develop mild symptoms, such as fever, headache, or body aches. In rare cases—approximately 1 in 150 infections—the virus can cause severe or life-threatening illness, particularly in individuals over 60 or those with chronic health conditions like cancer, diabetes, or hypertension.
“There are probably a higher number of total infections, many of which are not diagnosed,” noted Dr. David Sullivan, professor at the Johns Hopkins Bloomberg School of Public Health. However, he emphasized that the overall risk remains low: “This is still less than the chance of being hit by lightning.”
Currently, there is no specific treatment or vaccine available for West Nile virus. Supportive care is the primary approach for managing symptoms.
Eastern Equine Encephalitis (EEE)
Eastern equine encephalitis (EEE) is a rare but severe mosquito-borne disease that can be fatal. The virus cycles between mosquitoes and birds, particularly in freshwater hardwood swamp areas. Transmission to humans or horses typically requires a second mosquito species that feeds on both birds and mammals.
Most individuals infected with EEE do not experience symptoms. When symptoms do occur, they may include fever, chills, joint pain, and body aches. In more severe cases, the virus can cause encephalitis—dangerous inflammation of the brain and spinal cord—which can be fatal in about one-third of those affected.
“In worse cases, the virus infects brain tissue, leading to serious and sometimes permanent neurological damage,” explained Dr. Marc Siegel.
Children and older adults are more vulnerable to severe outcomes from EEE. Most U.S. cases are reported in Eastern and Gulf Coast states. As of 2025, the CDC has not recorded any new cases this year, and only 19 cases were reported in 2024.
There is no vaccine or specific antiviral treatment for EEE. Management focuses on supportive therapies for symptom relief.
Malaria
Malaria is a serious disease caused by a parasite transmitted by the Anopheles mosquito. Globally, it remains a major public health challenge. According to the World Health Organization (WHO), approximately 263 million malaria cases and nearly 600,000 deaths occurred worldwide in 2023, with the majority of cases in Africa.
Though malaria was eliminated in the U.S. in the 1950s, it still occasionally appears in individuals returning from regions where the disease is endemic. In 2020, the U.S. reported around 600 imported malaria cases. Notably, in 2023, several locally acquired malaria cases were confirmed in Florida and Texas—the first such cases in 20 years.
Symptoms range from mild flu-like illness to severe complications and can include fever, chills, headaches, and fatigue. Fortunately, prescription medications are available to treat and cure malaria, and antimalarial drugs can be used preventively. A malaria vaccine is also available in several countries with high transmission rates.
Dengue
Dengue is a rapidly spreading viral infection, with up to 400 million cases estimated globally each year, according to the CDC. It is endemic in many tropical and subtropical regions, but travel-related cases are also common in the United States.
As of 2025, 2,096 dengue cases have been confirmed in 39 U.S. jurisdictions, though most are believed to have been contracted abroad.
While many individuals remain asymptomatic, those who do develop symptoms may experience fever, rash, muscle or joint pain, nausea, vomiting, or eye pain. In some cases, the illness can become severe.
Currently, there is no antiviral treatment for dengue. Symptom management includes rest, fluids, and pain relief medications. A dengue vaccine is available in the U.S. for children aged 9 to 16 who have previously been infected and reside in areas where dengue is regularly transmitted.
Frequently Asked Questions
What is West Nile virus and how is it transmitted?
West Nile virus is a mosquito-borne virus primarily spread when mosquitoes bite infected birds and then bite humans. It cannot be transmitted from person to person.
What are the symptoms of West Nile virus?
Most people show no symptoms. Some may experience fever, headache, body aches, or rash. In rare cases, it can lead to severe neurological illnesses like encephalitis or meningitis.
How can I protect myself from mosquito bites?
Wear long sleeves and pants, apply mosquito repellent containing DEET or picaridin, avoid outdoor activity at dawn and dusk, and remove standing water near your home.
Is there a vaccine or treatment for West Nile virus?
There is currently no vaccine or specific antiviral treatment for West Nile virus. Treatment focuses on relieving symptoms and managing severe complications in hospital settings.
Are other mosquito-borne illnesses common in the U.S.?
Yes. Besides West Nile virus, illnesses like Eastern Equine Encephalitis (EEE), dengue, and rare malaria cases can occur, especially in warmer climates or due to international travel.
Who is most at risk for severe mosquito-borne illnesses?
Older adults (60+), people with weakened immune systems, and those with chronic conditions such as diabetes, cancer, or hypertension are at higher risk of complications.
Conclusion
Mosquito-borne illnesses like West Nile virus, Eastern Equine Encephalitis, dengue, and even malaria pose real health risks—especially as changing climates create more favorable conditions for mosquito populations to thrive. While many infections remain mild or asymptomatic, some can lead to severe or life-threatening complications, particularly among older adults and individuals with underlying health conditions.
The most effective way to protect yourself is through prevention: avoid mosquito-prone areas during peak hours, wear protective clothing, use EPA-approved repellents, and eliminate standing water around your home. Staying informed, prepared, and vigilant can significantly reduce your risk of infection and help keep your community safe.