Bird flu spreads to humans primarily through direct contact with infected birds, cattle, or contaminated environments. The virus travels in an animal’s saliva, mucus, and feces, and a person can become infected by breathing in airborne droplets or dust particles containing the virus, or by touching contaminated material and then touching their eyes, nose, or mouth. Since February 2024, the United States has reported 71 human cases of H5 bird flu and 2 deaths.
Direct Contact With Infected Animals
The most common route of infection is close, prolonged, unprotected exposure to sick birds or dairy cows. “Unprotected” means without respiratory or eye protection. Activities that generate the highest exposure include milking infected dairy cows, culling infected poultry flocks, and handling or defeathering infected birds. These tasks bring people into direct contact with large amounts of virus shed in respiratory secretions, saliva, and droppings.
You don’t have to touch an animal directly to be exposed. The virus can linger on surfaces like equipment, clothing, and tools. If you touch a contaminated surface and then touch your face, infection is possible. Liquid contaminated with live virus, such as raw milk from an infected cow, can also cause infection if it splashes into your eyes.
Airborne Spread
Bird flu virus can become airborne in droplets or fine dust particles, especially in enclosed spaces like barns and poultry houses where infected animals are shedding the virus. A person standing nearby can inhale these particles. The closer you are to an infected animal or contaminated environment, the higher the risk.
That said, airborne transmission appears to be relatively inefficient. NIH-funded research tested whether H5N1 could spread through the air between ferrets (a standard model for human flu transmission) housed in adjacent cages. Over nearly two weeks, none of the exposed ferrets developed symptoms or had detectable virus in nasal swabs, though one out of four did show an immune response, suggesting a low-level infection had occurred. This lines up with what we see in practice: most human cases trace back to direct contact, not casual airborne exposure.
Raw Milk and Food Safety
Raw, unpasteurized milk from infected dairy cows is a confirmed route of exposure. Lab studies have shown that mice become infected with H5N1 after consuming unpasteurized milk from an infected cow, and the findings suggest the same pathway could infect humans. No human cases from drinking raw milk have been confirmed so far, but the risk is real enough that health agencies flag it as a concern.
Cooking eliminates the virus. Poultry and eggs heated to an internal temperature of 165°F are safe. For ground beef, the target is 160°F, and whole cuts of beef should reach 145°F followed by a three-minute rest. No one in the United States has been infected by eating properly handled and cooked poultry products. A small number of infections in Southeast Asia have been linked to uncooked poultry or poultry blood.
How Migratory Birds Spread the Virus Globally
Wild birds, particularly waterfowl like ducks and geese, carry bird flu viruses across continents along established migration flyways. Research published in PNAS found that the timing of H5N1 outbreaks closely matches known bird migration routes, with the virus spreading at speeds that mirror the birds’ own travel pace (roughly 570 to 830 kilometers per month, depending on the flyway). The flyways act as corridors: the virus moves efficiently along them but rarely jumps between separate flyways, which limits how quickly it spreads to entirely new regions.
This is why outbreaks tend to follow seasonal patterns. As migrating birds arrive at stopovers and wintering grounds, they can introduce the virus to local poultry, other wild birds, and mammals. From there, it can spill into domestic livestock and eventually reach people who work closely with those animals.
Can It Spread From Person to Person?
Sustained human-to-human transmission of H5N1 has not been documented. Nearly every confirmed case traces back to animal exposure. However, the picture is more nuanced than a simple “no.” Investigations of household contacts of H5N1 patients have identified at least one case where a person with no contact with sick or dead poultry is believed to have been infected through human-to-human spread. Researchers at CIDRAP have noted that these findings challenge the long-held assumption that no human-to-human spread has ever occurred.
There’s also evidence that many infections may be mild or asymptomatic, meaning surveillance systems that focus only on severe illness could be missing cases. Of the 71 U.S. cases reported since early 2024, the majority (64) were detected through active monitoring of exposed workers rather than through people showing up sick at hospitals. This suggests the virus can infect people without making them seriously ill, which complicates efforts to track potential person-to-person spread.
Who Is Most at Risk
The people most likely to encounter bird flu are those with occupational or recreational exposure to animals. The highest-risk groups include:
- Poultry workers involved in farming, culling, or processing
- Dairy workers milking or handling infected cows
- Slaughterhouse workers handling live lactating dairy cattle, performing post-mortem inspections, or removing and transporting udders
- Veterinarians and animal health responders working with infected or potentially infected animals
- Wildlife rehabilitators, sanctuary workers, and zoo staff in contact with wild or captive birds
- Backyard flock owners and waterfowl hunters handling birds directly
Laboratory workers handling raw milk samples and food processing workers dealing with unpasteurized dairy products also face elevated risk.
How Long the Virus Survives in the Environment
H5N1 is a hardy virus outside a living host. CDC research tested its survival on polypropylene (common in plastic containers), stainless steel, and rubber surfaces, as well as in raw milk and wastewater. In wastewater at room temperature (about 72°F), the virus had a half-life of roughly half a day, but it took 16 days for a complete reduction to undetectable levels. Refrigeration slowed the virus’s decay significantly, meaning contaminated materials stored in cool environments remain infectious longer.
This environmental persistence matters for anyone working in spaces where infected animals have been. Barns, milking equipment, transport vehicles, and water sources can harbor live virus for days, creating ongoing exposure risk even after sick animals have been removed.