West Nile virus primarily affects birds and is spread via mosquito bites. Yet the virus can make the jump to humans when infected mosquitoes bite us. Though humans aren't typically symptomatic, roughly one in five infected people feel sick from the virus and rarely it causes severe life-threatening brain health complications, including encephalitis and meningitis.
The virus was first identified in the blood of an infected human in Uganda in 1937, and soon hopscotched to Europe. It wasn't detected in the US until the summer of 1999, when it caused a human outbreak and killed nearly 5,500 crows in just four months. The virus has spread widely and is found in 48 US states. Beyond humans, birds, and mosquitoes, the virus has mostly been detected in horses.
There are vaccines to protect birds and horses, but none for humans. The lack of market for a human vaccine—since so many people are asymptomatic—is one major development hurdle. But in 2025, the US experienced a 40% spike in its West Nile human caseload, due in part to warmer temperatures and increased rainfall conditions. More than 1,400 people had severe infection complications.
Hours of research by our editors, distilled into minutes of clarity.
Written by orca trainers who went on to become scientists, it states mosquitoes bite orcas at night when the animals hang out close to the surface. What's more, antibiotics used to keep captive orcas healthy also may depress their immune response, boosting the risks of them being unable to fight off diseases like West Nile Virus and encephalitis, which have killed orcas at US water parks.
Mosquitoes pick up the virus when they feed on birds, and then pass it to other birds with their bite, delivering it via virus-laden saliva. Hundreds of different North American bird species can harbor the virus—even if they don't all get sick from it.
In 1937, scientists isolated the virus from a woman in the West Nile District of Uganda. It's since been detected in much of Africa, Eastern Europe, West Asia, the Middle East, and was first detected in US patients during an outbreak in 1999.
Tracking numbers from the CDC indicate that the virus spread widely that year, with more than 2,000 reported human cases and many states seeing more than 25 sickened people. More than 1,400 people had neuroinvasive cases of the disease—meaning the virus had entered the nervous system, which can cause severe, potentially life-threatening illness.
About 1 in 5 infected people develop symptoms from West Nile, including symptoms such as fatigue, head and body aches, and vomiting. But around 1% of patients develop severe complications such as encephalitis or meningitis, which can be life-threatening. Those who are most likely to become severely ill are people over age 60.
Fragments of the virus were detected in mosquitoes that were being collected in Britain for the first time in 2023, but the UK government considers the overall risk to people to be low. As of spring 2026, there have never been detected locally-acquired cases of the disease.
This catchy rap from biologist MC Bugg-Z and the Fairfax County government in Virginia walks people through prevention steps and the dangers of the virus, telling listeners to "rock" insect repellant and talking about the history of the disease in the US.
The animals can contract the virus via infected mosquito bites, but for horses there are four vaccines available. Though West Nile's often asymptomatic or mild in horses, the disease can sometimes be deadly, killing via severe neurological inflammation. There are no reported instances of people becoming sickened by infected horses or vice-versa.
A major hurdle for West Nile vaccine development is the lack of a market for it since so many people are symptomatic for the infections. Still, a growing number of infections, including a 40% spike in 2025, may change that calculation.
The condors became endangered in the 20th century primarily due to lead poisoning from consuming animals shot with lead bullet fragments inside them, so condors in captivity and in the wild are vaccinated against diseases including highly pathogenic avian flu and West Nile.
Discover a broad spectrum of topics, from fundamental anatomy to the latest in medical research and treatments, all aimed at enhancing your understanding of health, wellness, and our bodies. Our resources are meticulously curated to offer precise and pertinent information to students, professionals, and anyone interested in learning more about the medical sciences. Dive in to stay informed, make well-informed health decisions, and explore the intriguing world of medicine.