How a scientist discovered West Nile is spread by birds

Listen to Tracey McNamara, former chief pathologist at the Bronx Zoo, detailing her surprising late 1990s discovery that established West Nile virus is spread by birds. Financing animal disease surveillance studies, she says, saves human lives and surveillance gaps may fuel missed opportunities to identify dangerous diseases before they jump between species.

Protecting yourself from West Nile involves avoiding getting bites and clearing standing water.

Hear from a NYC public health official who led the first US investigation into West Nile about the challenges investigators faced and their working theories about that outbreak.

Robins are a major host for West Nile in North America, even though other species more readily spread the disease.

West Nile Virus is maintained in nature, even without human infections

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.

There's no cure for West Nile virus—and infection can leave patients with permanent symptoms.

US West Nile case numbers were up 40% in 2025 across the United States though theories vary to explain this surge.

West Nile was reported in 47 states in 2025, with more than 1,400 people severely affected

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.

West Nile virus found in the UK for the first time

The West Nile virus, which mainly spreads between birds and mosquitoes, was detected in the UK for the first time. While no evidence exists for any human infections, climate change may be expanding mosquitoes—and the diseases they carry—farther north.

West Nile is named after the district in Uganda where it was first isolated from a patient

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.

Dead birds are often the warning sign health officials look for as a sign of West Nile danger.

Using mosquitoes to vaccinate bats could curb the spread of deadly diseases.

West Nile virus was detected in UK mosquitoes for the first time in 2023

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.

Long-term memory issues among West Nile survivors may be from the body's immune response, not the viral infection.

Powassan virus, a rare tick-borne disease named after the Canadian town where it was discovered in 1958, is on the rise in the northeastern United States, and it kills roughly 10% to 15% of patients with symptomatic disease.

Most cases of West Nile virus are asymptomatic in people

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.

West Nile has been detected in at least 2 species of reptiles, including American alligators.

Horses have four licensed West Nile vaccines—but humans have none

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.

Scientists found that if cats ate infected rodents, they could get West Nile in lab experiments.

Nearly 5,500 crows died during four months in 1999 from West Nile in NYC.

Horses can—rarely—get West Nile, too

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.

Wolbachia: the bacteria that could eliminate dengue fever

Dengue fever, transmitted by mosquitoes, kills more people annually than all types of natural disasters combined. However, mosquitoes that host Wolbachia bacterium have their ability to transmit the virus naturally suppressed, offering a route to eliminating nearly 25,000 deaths annually.

Horses account for about 96% of known mammalian cases of West Nile virus infection beyond humans.

Bats have evolved to carry viruses without getting sick

Bats—the likely animal reservoir for diseases including Ebola and coronavirus—tolerate viruses without getting sick. The secret lies in evolutionary adaptations allowing bats to become the world’s only flying mammal and to thrive under immense physiological stressors.

Bats are uniquely suited to act as reservoirs of viruses.

Some of the deadliest diseases to jump from animals to humans originate in bat species. This rich visualization explores the diversity of bats and the factors that make them efficient vectors for harboring and transmitting disease, from their long lifespans to their migration patterns.

Wolbachia is a naturally occurring bacterium being employed to fight dengue

Scientists are fighting dengue spread in a roundabout way: injecting bacteria into Aedes aegypti mosquito eggs, which blocks the dengue virus from replicating in the tissue of mosquitoes. Researchers hope the bacterium will be passed to future generations, inoculating them against dengue. Wolbachia is also being investigated to combat other mosquito-borne diseases.

Wolbachia is also being used to slash mosquito numbers

The naturally occurring bacterium does not naturally infect Aedes aegypti mosquitoes, but communities in locations including California and Texas have released male mosquitoes infected with Wolbachia and reported a significant drop in subsequent mosquitoes: When infected males mate with uninfected wild females, the eggs don't hatch.

'Sloth fever' is an emerging health threat, especially to pregnant women

This disease, caused by the Oropouche virus, is found in animals including sloths, primates, and birds. It primarily spreads through the bite of midges—insects commonly called "no-see-ums." It's endemic to the Amazon region, but US travelers are increasingly contracting it internationally.

Wearing tick repellent, covering up, and performing tick checks are the best Lyme prevention

Spraying repellents onto your clothes—and reapplying them—or donning impregnated bug-repelling clothes can help keep ticks away. Effective sprays include DEET, picaridin, or permethrin. Bug spray applied to the body should be applied on top of sunscreen, and only after the sunscreen has dried.

Explore inside a mosquito factory where millions of mosquitoes are bred weekly

In Medellin, Colombia, scientists are infecting Aedes aegypti mosquitoes with Wolbachia, a bacterium that wouldn't otherwise infect the species, but does block its ability to transmit dengue, Zika, yellow fever, and chikungunya. In the wild, infected mosquitoes would breed with wild mosquitoes, pass down Wolbachia, and thus reduce disease transmission.

How do vaccines work? Watch this brief 101

Watch this under-3-minute video explaining how vaccines use the body's immune system to prepare our bodies against viral and bacterial threats to our health.

More than 20 mosquito-borne and tick-borne diseases are found in the US

Children, older adults, and people with compromised immune systems are at particular risk of poorer health outcomes from these diseases. But dog owners and outdoor enthusiasts are among those at particular risk of contracting these illnesses, which include Lyme disease, Zika, and the condition Alpha-gal Syndrome.

Scientists still don't know why DEET is such an effective bug repellent

This chemical, used to keep ticks, fleas, mosquitoes, and other nuisance bugs away, is used by an estimated one-third of the US population and has been available to consumers since 1957. Researchers theorize that multiple mechanisms may be at work to deter bugs, including perhaps masking or confusing bugs' olfactory receptors.

Measles, eliminated in the US in 2000 but now making a resurgence, is one of the most contagious preventable diseases.

Rabies kills more than 50,000 worldwide each year

Rabies transmission to humans typically comes from an infected animal, and death was very common before recent protocols were developed (post-exposure prophylaxis). The virus is neurotropic, which means it has an affinity for nerve tissue, and moves quickly from broadly infecting the body to destroying the brain.

Scientists have struggled to pin down Ebola's natural reservoir species for decades, but the available evidence suggests that it's very likely fruit and insect-eating bats found in Africa.

What if we killed every mosquito on Earth?

Mosquitoes aren't just annoying, they're deadly. they're also responsible for an estimated 725,000 deaths worldwide, every year. Would it be so bad if we found a way to wipe them out completely? Real scientists have gamed out this hypothetical scenario, concluding it actually wouldn't be that bad. This five-minute video breaks down one way this could happen.

Rabies is nearly 100% fatal once symptoms appear

Rabies, a viral disease attacking the central nervous system, is nearly always fatal once symptoms appear. It spreads through the infected animal saliva (typically through bites) and causes encephalitis, leading to severe neurological symptoms and death. See how the lyssavirus—the bugs behind rabies—work once inside the body.