West Nile virus is not contagious from person to person through casual contact. You cannot catch it by touching, kissing, coughing near, or caring for someone who is infected. The virus spreads almost exclusively through mosquito bites, and humans are what scientists call “dead-end hosts,” meaning the virus essentially stops with us. Unlike birds, people don’t develop high enough levels of virus in their bloodstream to pass it back to a biting mosquito, let alone to another person.
How the Virus Actually Spreads
West Nile virus cycles between Culex species mosquitoes and birds in the wild. A mosquito feeds on an infected bird, picks up the virus, then bites another bird, keeping the cycle going. Humans enter the picture accidentally. When an infected mosquito bites a person, the virus can cause illness, but the chain of transmission ends there. The virus level in human blood stays too low for a mosquito to pick it up during a subsequent bite and carry it to someone else.
Horses and other mammals are also dead-end hosts. The virus doesn’t spread between animals and people through contact either.
Rare Exceptions: Medical Transmission
There are a handful of documented ways West Nile virus has passed between humans, all involving medical procedures. In 2002, transmission through blood transfusion and organ transplantation was confirmed for the first time in the United States. That discovery led to nationwide screening of blood donors, which remains in place today.
Organ donors, however, are not routinely screened for West Nile virus. In a 2008 Louisiana case, a heart transplant recipient developed severe neurological disease after receiving an organ from a donor who had unknowingly been infected through a blood transfusion. Transplant recipients are particularly vulnerable because their immune systems are deliberately suppressed to prevent organ rejection.
There is also a single documented case suggesting transmission through breast milk. In 2002, a woman developed West Nile encephalitis from a blood transfusion shortly after giving birth. Lab analysis found evidence of the virus in her breast milk, and her infant later tested positive. The baby had minimal outdoor exposure, making mosquito transmission unlikely. Importantly, the infant never developed symptoms and remained healthy. The CDC considers the risk of breastfeeding transmission very low and does not recommend stopping breastfeeding because of a West Nile infection.
What Most Infections Look Like
The incubation period after a mosquito bite is typically 2 to 6 days, though it can stretch to 14 days and may run even longer in people with weakened immune systems. The majority of people infected with West Nile virus never feel sick at all. About 1 in 5 develop West Nile fever, which involves headache, body aches, fatigue, and sometimes a rash. These symptoms generally resolve on their own.
A much smaller fraction, roughly 1 in 150 infections, leads to neuroinvasive disease: encephalitis (brain inflammation) or meningitis (inflammation of the membranes surrounding the brain and spinal cord). This is the serious form. Symptoms include high fever, neck stiffness, disorientation, tremors, and in severe cases, paralysis or coma. Neuroinvasive disease is more common in older adults and people with compromised immune systems, and recovery can take weeks to months.
West Nile in the U.S. Today
West Nile virus remains the most common mosquito-borne illness in the continental United States. In 2025, there were 2,076 reported human cases across 47 states. Of those, 1,434 were neuroinvasive, meaning the virus affected the brain or spinal cord. Colorado reported the most cases (285), followed by Illinois (149), Texas (127), and Minnesota (122). Cases tend to peak in late summer and early fall, when mosquito populations are highest.
How Testing Works
If you develop symptoms consistent with West Nile virus, especially during mosquito season, a blood test can check for antibodies your body produces in response to the virus. These antibodies typically become detectable 3 to 8 days after symptoms start and can persist for 30 to 90 days, sometimes longer. If blood is drawn very early in the illness, a negative result doesn’t necessarily rule out infection, and the test may need to be repeated.
For people with neurological symptoms, testing of spinal fluid may also be performed. There is no specific antiviral treatment for West Nile virus. Care focuses on managing symptoms, with hospitalization reserved for severe neuroinvasive cases.
Preventing Mosquito Bites
Since mosquitoes are the primary route of infection, prevention comes down to avoiding bites. EPA-registered insect repellents containing DEET, picaridin, IR3535, or oil of lemon eucalyptus are all proven effective and safe, including for pregnant and breastfeeding women when used as directed.
Clothing matters too. Loose-fitting long sleeves and pants create a physical barrier, and treating clothing with permethrin (an insecticide that survives multiple washes) adds another layer of protection. Permethrin should not be applied directly to skin.
Around your home, the most effective step is eliminating standing water where mosquitoes breed. Check for water-holding containers weekly: flowerpot saucers, old tires, buckets, birdbaths, kiddie pools, and clogged gutters. Empty and scrub them, or toss what you don’t need. Keeping window and door screens in good repair and using air conditioning when possible also reduces indoor exposure.