How worried should you be about West Nile virus?
Dr. Justin Remais on how to protect yourself from this mosquito-borne disease
Dr. Justin Remais began studying West Nile virus following record breaking numbers of infections in California in 2014 and 2015. His team began piecing together the factors contributing to West Nile activity in neighborhoods across Southern California, predicting that changes to California’s climate might contribute to greater transmission in future decades, especially in coastal communities.
Now, as professor and chair of the Division of Environmental Health Sciences at the UC Berkeley School of Public Health, Dr. Remais is noting another sharp rise in California’s West Nile virus activity. The state has recorded more than 250 dead birds that tested positive for West Nile virus as of mid-July, a striking increase from just 50 recorded last year at this time. In Los Angeles County, there were 67 positive mosquito pools – a group of captured mosquitos that tested positive for the virus – compared to just 6 at this same time in 2025, a tenfold increase. Counties in Southern and Central California have recorded five human cases by mid-July, compared to three in 2025. We asked Professor Remais about West Nile virus, and what the public can do to protect themselves from this disease.
UCBPH: What is West Nile Virus? Why is it dangerous?
Dr. Justin Remais: West Nile virus is the leading cause of mosquito-borne disease in the contiguous United States. It is spread to people through the bite of an infected mosquito. While most people who become infected do not develop symptoms, the virus can cause severe, sometimes fatal, neurological illnesses, such as encephalitis (inflammation of the brain) or meningitis (inflammation of the membranes surrounding the brain and spinal cord). Less than 1% of infected people develop severe neurologic disease, which can result in long-term disability or death.
How is it transmitted to humans, and how was it discovered?
West Nile virus is transmitted to people predominantly through the bites of infected mosquitoes, especially the Culex species. Mosquitoes become infected when they feed on infected birds, including robins, jays, and crows. The virus is picked up by the mosquito and is transmitted in its saliva when it bites a person or another animal.
Scientists believe the virus has likely been in the United States since or before 1999 when it was first identified in several eastern states, though it was originally discovered in Africa in the West Nile district of Uganda. The virus is thought to have first arrived in the Los Angeles metropolitan area in 2003. In a very small number of cases, the virus has spread through blood transfusions and organ transplants, and from mother to baby through breastfeeding and during pregnancy.
The CDC says that the current U.S. West Nile virus season is off to a historic 22-year high, with 91 human cases confirmed nationally this year as of July 21, 2026, according to tracking data reported by the CDC ArboNET Surveillance System. Why are we seeing cases of West Nile virus now? Where are most cases concentrated in California? How is West Nile virus connected to climate change?
Because the spread of West Nile virus is closely linked to mosquitoes—which cycle in number with the seasons—it is considered a seasonal disease that increases in incidence in the summer and continues into the fall.
In California, cases are often first recorded in late spring and extend into late September in Northern California, October in the Central Valley, and November in Southern California. Geographically, cases in California are often concentrated in regions like the Central Valley and Southern California, where the interplay between local mosquito populations, bird hosts, and favorable environmental conditions creates areas of elevated risk.
Our research at UC Berkeley has shown that transmission of West Nile virus responds profoundly to temperature. We found that infection with the virus among captured mosquitoes in the Los Angeles metropolitan area was strongly associated with the average temperature in the neighborhood.
Our data revealed a sharp transition where—as temperatures shift between 70 to around 73 degrees Fahrenheit—the likelihood of capturing infected mosquitoes in Southern California neighborhoods increases sharply. Above this range, conditions become consistently favorable for transmission of the virus.
This explains why certain coastal Southern California communities—where typical summer conditions often hover right at the boundary between favorable and inhibitory temperatures—seem to be protected some years, yet highly vulnerable in others.
With significant warming expected over the coming decades, a greater number of West Nile cases may be expected along the Southern California coast as climate change pushes coastal climates more consistently into the favorable zone for transmission. This increase is driven not just by single hot days, but by sustained warm temperatures over the course of weeks that give mosquitoes time to acquire the infection and pass it on to bird host species, and also people.
What are the first symptoms, and how does it affect the body? Can it be transmitted from human to human?
More than three-quarters of people infected with West Nile virus will not show any symptoms. For those who develop mild illness (sometimes called West Nile fever), the first symptoms typically include a fever, headache, body aches, joint pains, nausea, vomiting, diarrhea, and occasionally a rash. These symptoms usually start 2-6 days after a bite by an infected mosquito. Most people recover completely, but fatigue and weakness can last for weeks or months. In severe cases—affecting about 1 in 150 people—the virus impacts the central nervous system, causing serious conditions like encephalitis or meningitis. Symptoms can include high fever, severe headache, neck stiffness, stupor, disorientation, muscle weakness, convulsions, vision loss, numbness, and paralysis.
Can I get it from birds who come to my fountain or feeder?
No, you cannot get West Nile virus directly from touching or being near live birds at your fountain or feeder. The virus requires a mosquito vector to transmit it from an infected bird to a person. You should consider eliminating standing water in your yard (including regular flushing of bird baths) to prevent mosquitoes from breeding. If you find a dead bird—especially crows, jays or other corvids—you can report it to the California Department of Public Health. Dead birds can be an early indicator of West Nile virus activity in your community, and can help inform local mosquito control efforts.
Who is at risk for West Nile? Can a mother transmit it to the fetus? Or through breast milk?
Anyone who lives in or visits areas where West Nile virus is present is at risk of infection—West Nile virus has been reported in most U.S. counties. However, people aged 60 years and older have the highest risk of developing a severe infection. People with certain underlying medical conditions—such as cancer, diabetes, hypertension, kidney disease, or immunocompromised individuals—are also at a significantly higher risk for severe illness.
While people do not generally spread the virus, in a very small number of rare cases, West Nile virus has spread from mother to baby during pregnancy, delivery, or through breastfeeding. Pregnant women and nursing mothers should talk to their doctor if they develop symptoms, but they should primarily focus on taking precautions to avoid mosquito bites.
How do I protect myself? Are there safe insect repellants, and can I use them if I’m pregnant or nursing?
The best way to protect yourself is to avoid mosquito bites. This involves reducing your time outside during peak mosquito hours (dusk and dawn), wearing long, loose-fitting shirts and pants, and making sure windows and doors have tight-fitting screens to keep mosquitoes outside.
Additionally, you should drain any standing water around your home where mosquitoes like to breed, such as buckets, flower pots, pet bowls, birdbaths, and clogged gutters.
For repellents, use a product registered with the Environmental Protection Agency (EPA), such as those containing DEET, Picaridin, IR3535, or Oil of Lemon Eucalyptus. When used as directed, EPA-registered insect repellents are proven safe and effective, including for pregnant and breastfeeding women. The CDC provides excellent guidance on the best ways to prevent mosquito bites.
Are vaccines available to prevent it?
While highly effective vaccines are approved to reduce the risk of infection in horses, no vaccines or specific medicines are currently licensed for use in humans to prevent or treat West Nile virus disease. Clinical management of human cases is purely supportive. Mild illness is usually managed with rest, hydration, and over-the-counter pain relievers, while severe cases can require hospitalization for supportive care.
What should I do if I think I have it, or if someone in my family does? Should I report it somewhere, or will my health care provider do that?
If you have mild symptoms, they typically pass on their own, and you can often manage them with rest, hydration, and over-the-counter pain relievers. However, if you or a family member develop symptoms of severe West Nile virus illness—such as an unusually severe headache, high fever, neck stiffness, confusion, or muscle weakness—you should seek medical attention immediately. You do not need to report the infection yourself. West Nile virus is a mandatory notifiable disease—if your doctor diagnoses you with West Nile virus, state and territorial health departments are automatically notified and are required to report the case to the CDC in order to monitor and respond to the changing epidemiology of the disease.
