Avian flu, also called bird flu, is a type of influenza A virus that naturally circulates among wild birds and can infect domestic poultry, other animals, and occasionally humans. Since 2003, more than 23 countries have reported over 890 human infections with the most dangerous strain, H5N1, to the World Health Organization. While human cases remain rare, the virus has drawn increasing attention as it spreads to new animal hosts, including dairy cattle in the United States.
How the Virus Is Classified
Influenza A viruses are sorted by two proteins on their surface: hemagglutinin (H) and neuraminidase (N). In birds, 16 versions of the H protein and 9 versions of the N protein have been identified, which combine into a wide range of subtypes. That’s where names like H5N1 and H7N9 come from.
These subtypes fall into two categories based on how severe they are in poultry. Low pathogenicity strains cause mild illness in chickens, things like ruffled feathers or a dip in egg production. Highly pathogenic strains cause severe disease and high death rates in infected flocks. Only certain H5 and H7 subtypes are classified as highly pathogenic, though most H5 and H7 viruses circulating in wild birds are actually the milder form. Five subtypes (H5, H6, H7, H9, and H10) have caused human infections, with H5, H7, and H9 being the most common.
How It Spreads
Wild waterfowl, especially ducks and geese, are the natural carriers. They often show no symptoms but shed the virus in their droppings, saliva, and nasal secretions. From there, the virus can reach domestic poultry either through direct contact or, more commonly, through contamination of the farm environment. Virus particles can hitch a ride on feed, equipment, boots, and clothing.
Farm-to-farm movement is another major route. The U.S. Department of Agriculture has identified this as a significant driver of outbreaks, which is why biosecurity on poultry farms is considered the single most important prevention measure. Workers entering and leaving facilities can unknowingly carry the virus on their gear if protocols aren’t followed.
Human infections typically happen through close, prolonged contact with infected birds or contaminated environments. The virus does not spread easily between people. The biological reason: avian flu viruses prefer a type of receptor found deep in the human airway, while seasonal flu viruses target receptors in the upper airway, nose, and throat. This mismatch makes it harder for bird flu to infect human cells and much harder for it to pass from one person to another. Some strains have shown signs of adapting to human-type receptors, which is why scientists monitor these viruses closely.
Symptoms in Humans
The illness ranges enormously in severity. On the mild end, symptoms can look like a typical flu: fever (100°F or higher), cough, sore throat, runny nose, muscle aches, headaches, and fatigue. Eye redness or irritation (conjunctivitis) is a notable symptom that sets bird flu apart from regular seasonal flu. Some people with confirmed infections have had no symptoms at all.
Severe cases involve high fever, shortness of breath, difficulty breathing, altered consciousness, and seizures. Complications can escalate to pneumonia, respiratory failure, acute kidney injury, multi-organ failure, sepsis, and inflammation of the brain. The case fatality rate for H5N1 infections reported globally has been high historically, though this number is likely inflated because mild cases often go undetected and unreported.
Diagnosis and Treatment
Bird flu cannot be diagnosed based on symptoms alone since it mimics other respiratory illnesses. The preferred lab method is a specialized PCR test that detects viral genetic material from respiratory samples. Standard rapid flu tests used in doctor’s offices are not reliable for identifying bird flu specifically. If your doctor suspects exposure, samples are typically sent to a public health laboratory for confirmation.
Antiviral medications are the main treatment, and timing matters enormously. Starting treatment within two days of symptom onset is associated with significantly lower mortality. An observational study of H5N1 patients in Indonesia found that early treatment (within two days) was linked to much better survival compared to treatment started five or six days after symptoms appeared. For hospitalized patients, treatment is recommended regardless of how much time has passed since symptoms began, because it can still offer some benefit.
The Dairy Cattle Connection
In 2024, H5N1 was detected in U.S. dairy cattle herds, a development that caught many scientists off guard. The virus was found in raw milk from infected cows, raising questions about food safety.
The reassuring finding: commercial pasteurization effectively destroys the virus. The FDA tested 464 pasteurized dairy products, including milk, cheese, butter, and ice cream, and all came back negative for viable H5N1. In lab experiments designed to simulate commercial processing, heating milk to 161°F (72°C) for 15 seconds completely inactivated the virus in every trial. The process eliminates roughly 1 trillion virus particles per milliliter, a massive margin of safety.
Raw milk is a different story. The standard 60-day aging process used for raw milk cheese does not eliminate the virus. H5N1 survived through and beyond the full aging period in research from Cornell University. The FDA now recommends that dairy processors avoid using milk from infected cows for any raw milk products. For consumers, pasteurized dairy remains safe.
Prevention for Those at Risk
Most people face very little risk of bird flu. The groups who need to take precautions are poultry workers, farmers, veterinarians, and anyone involved in culling or handling sick birds. The CDC recommends layered protective equipment based on exposure level. For high-exposure settings like depopulation of infected flocks, this includes a respirator, fluid-resistant coveralls, safety goggles, boot covers, head covers, and disposable gloves.
Farms handling outbreaks establish “clean” and “dirty” zones. Workers put on protective gear in the clean area and remove it in a separate dirty area, moving in one direction from contaminated to clean spaces. Contaminated clothing and equipment stay on-site. Workers shower before leaving and change into uncontaminated clothes. These protocols also mean no eating, drinking, or touching phones while wearing protective gear in contaminated zones.
For the general public, the practical advice is straightforward: avoid direct contact with wild birds, sick or dead poultry, and surfaces contaminated with bird droppings. Don’t drink raw milk. Cook poultry and eggs thoroughly, as heat destroys the virus.
Vaccines and Preparedness
There is no widely available bird flu vaccine for the general public. However, the U.S. government has maintained stockpiles of “prepandemic” vaccines targeted against circulating H5N1 strains since 2005. The idea behind these stockpiles is to provide some level of protection quickly if a pandemic strain emerges, buying time while a more precisely matched vaccine is manufactured. Domestic production capacity has been built with the goal of vaccinating the entire U.S. population within six months of a pandemic declaration. These are contingency measures, not something most people need to think about right now, but they represent a significant layer of preparedness if the virus evolves to spread efficiently between people.