Bird flu spreads primarily through direct contact with infected birds or their contaminated environments. The virus lives in both the respiratory and gastrointestinal tracts of birds, meaning it sheds through saliva, nasal secretions, and feces. More than 100 wild bird species carry avian influenza A viruses, and those birds seed outbreaks in domestic poultry, dairy cattle, and occasionally humans.
How It Spreads Between Birds
Wild waterfowl like ducks and geese are the natural reservoir. Many carry the virus without showing symptoms, shedding it in large quantities through droppings. Other birds pick it up through three main routes: direct contact with an infected bird, breathing in airborne droplets or dust, and exposure to contaminated environments like shared water sources, feed, or soil.
That last route is especially significant. Avian influenza is remarkably durable in the environment. At room temperature (around 20°C), the virus survives roughly 18 days in soil and 21 days in water. In cold conditions near 4°C, it persists up to 60 days on metal surfaces, 120 days in contaminated soil and litter, and 40 days in seawater. Frozen in ice at minus 20°C, it can remain viable for up to a year. This is why outbreaks often flare during fall and winter migration seasons, when cold temperatures preserve the virus in the ponds and wetlands where birds congregate.
Once the virus enters a poultry flock, it can tear through a barn rapidly. Shared equipment, contaminated boots, and vehicle tires all act as vehicles. Highly pathogenic strains like H5N1 can kill nearly an entire flock within days.
How It Jumps to Mammals
Bird flu viruses are built to infect birds, not mammals. The key reason is receptor compatibility. Influenza viruses latch onto sugar molecules called sialic acid receptors on the surface of cells. Bird flu viruses prefer a specific configuration of these receptors (the alpha-2,3 linkage) that lines the intestinal and respiratory tracts of birds. Human airways are dominated by a different configuration (alpha-2,6), which is why bird flu rarely gains a foothold in people.
When it does cross into mammals, it typically requires heavy, prolonged exposure. The virus enters through the eyes, nose, or mouth, either from inhaling contaminated dust and droplets or from touching a contaminated surface and then touching your face. In the United States, infections in people have been linked to close contact with infected poultry or dairy cows, not casual proximity.
The Dairy Cattle Connection
Since 2024, H5N1 has been detected in U.S. dairy herds, adding a new transmission pathway. Infected cows carry high concentrations of the virus in unpasteurized milk. This raises particular concern because milking equipment shared between animals can shuttle the virus across a herd, and raw milk itself becomes a potential exposure route for farmworkers and anyone who consumes it unpasteurized.
Cattle respiratory tracts have a different distribution of sialic acid receptors than pigs or humans. The mammal-type receptors in cows are mostly buried in deeper tissue layers rather than lining the surface of the airway, which may explain why cattle infections look different from what scientists initially expected. Still, every new mammalian host gives the virus additional chances to adapt.
Why Human-to-Human Spread Remains Rare
Sporadic bird-to-human infections have occurred for decades, but sustained human-to-human transmission has not. The virus simply isn’t efficient at binding to human airway cells or replicating well once inside them. For that to change, specific genetic mutations would need to accumulate.
Researchers at Scripps Research identified one such mutation, called Q226L, that significantly improves the virus’s ability to attach to human-type receptors. But binding is only part of the equation. Additional changes, particularly in a protein that helps the virus replicate inside cells (the PB2 protein, specifically the E627K mutation), would likely be necessary before H5N1 could spread efficiently from person to person. Scientists actively monitor circulating strains for exactly these mutations.
The concern is that every infection in a mammal, whether a dairy cow, a pig, or a human farmworker, gives the virus a chance to acquire these adaptations through random mutation or by swapping genetic material with seasonal flu strains already circulating in mammals.
Who Faces the Highest Risk
Most people have essentially zero exposure to bird flu in daily life. Risk concentrates among people who work directly with animals or animal products. The CDC identifies poultry workers and dairy workers as the groups currently most likely to encounter the virus. Specific high-risk activities include milking infected cows, culling infected flocks, and handling or defeathering infected birds.
Beyond farm settings, the risk extends to slaughterhouse workers handling lactating dairy cattle, veterinarians, wildlife rehabilitators, fur farm workers, zoo and aquarium staff, and public health responders involved in outbreak control. Recreational exposures matter too: backyard flock owners and hunters of wild waterfowl both face elevated risk compared to the general public.
Incubation Period and Early Symptoms
If you are exposed and become infected, respiratory symptoms typically appear about three days later, though the window ranges from 2 to 7 days. Eye symptoms like redness and irritation can show up faster, within one to two days of exposure. This is worth knowing because conjunctivitis has been the primary symptom in several recent U.S. cases among farmworkers, sometimes without any respiratory illness at all.
Food Safety and Practical Precautions
Cooking poultry and eggs to an internal temperature of 165°F kills avian influenza viruses. No one in the United States has been confirmed infected from eating properly handled and cooked poultry. A small number of infections linked to uncooked poultry or poultry blood have occurred in Southeast Asia, which is why food safety agencies emphasize keeping raw poultry separate from ready-to-eat foods and cooking all poultry products, including eggs, thoroughly before eating.
Pasteurization effectively eliminates the virus from milk. The risk from dairy products comes specifically from unpasteurized (raw) milk, which can contain high viral concentrations when sourced from infected herds. Commercial pasteurized milk remains safe.
For anyone who keeps backyard chickens, hunts waterfowl, or works around livestock, the practical priorities are straightforward: avoid touching sick or dead birds with bare hands, wash hands thoroughly after any animal contact, don’t drink raw milk, and keep birds away from your face. If a bird in your flock dies unexpectedly, contact your state veterinarian or local agricultural extension office rather than handling the carcass yourself.