Bird flu in humans is an infection caused by avian influenza A viruses that normally circulate in wild birds and poultry but occasionally jump to people. These infections are uncommon, but when they do occur, certain strains carry case fatality rates of 40% to 50% in hospitalized patients. Most human cases result from direct contact with infected birds or other animals rather than spread between people.
Which Strains Infect Humans
Not all bird flu viruses pose the same threat to people. Two strains have caused the vast majority of documented human infections: H5N1 and H7N9. H5N1 has infected more than 925 people across 24 countries since 1997, with roughly half of those cases proving fatal. H7N9, first detected in China in 2013, infected more than 1,500 people during epidemics between 2013 and 2017, killing about 40% of hospitalized patients.
Other strains cause human infections less frequently. H5N6 has been reported in more than 90 people since 2014, mostly in China, with a fatality rate above 50%. H9N2, a lower-severity strain, and H5N8 have also infected small numbers of people. The strain matters because it determines how severe the illness is likely to be and how public health agencies respond.
How People Get Infected
The vast majority of human bird flu cases trace back to contact with infected animals. Visiting a live poultry market, handling sick or dead birds, and working on farms with infected flocks are the most common exposure routes. People become infected after breathing in respiratory droplets, saliva, or tiny dust particles containing the virus, or after touching contaminated surfaces and then touching their eyes, nose, or mouth.
The current outbreak in the United States has added a newer exposure route. Between March 2024 and May 2025, 70 human H5N1 cases were confirmed across 13 states. Of those, 41 (59%) involved exposure to infected dairy cows, 24 (34%) to commercial poultry (mostly during depopulation activities), and 2 to backyard flocks. Three cases had no identified source. Raw milk from infected cows can carry live virus, and splashing contaminated liquid into the eyes is a documented infection route.
Person-to-person spread remains extremely rare. No human-to-human transmission has been identified in the United States. In other countries, limited spread has occurred only after prolonged, close contact without protective equipment, such as a family member caring for someone severely ill with bird flu.
Symptoms and How They Progress
Bird flu symptoms in people overlap significantly with regular flu, which is part of what makes early identification tricky. Mild cases can include eye redness or irritation (conjunctivitis), low-grade fever at or above 100°F, cough, sore throat, runny or stuffy nose, muscle aches, headaches, and fatigue. Some people experience diarrhea, nausea, or vomiting, though these are less common. In the recent U.S. dairy cow cases, conjunctivitis was frequently the primary or only symptom.
Severe cases look very different. High fever, shortness of breath, difficulty breathing, altered consciousness, and seizures signal that the infection is progressing. Complications can include pneumonia, respiratory failure, acute kidney injury, multi-organ failure, septic shock, and inflammation of the brain. The progression from mild to severe can happen quickly, which is why early antiviral treatment is critical for anyone with a known exposure.
How Bird Flu Is Diagnosed
Standard flu tests alone cannot confirm bird flu. Diagnosis follows a multi-step process at public health laboratories. A specimen is first tested for influenza A. If positive, it undergoes subtyping to rule out seasonal strains like H1N1 and H3N2. If the virus is influenza A but doesn’t match any seasonal subtype, it gets tested specifically for H5.
The type of sample collected depends on symptoms. If you have only eye symptoms, doctors collect both an eye swab and a nasal swab separately for parallel testing. If you have respiratory symptoms, a combination of nasal and throat swabs goes into a single transport tube. Any positive result at a state lab is considered “presumptive positive” and gets sent to the CDC for final confirmation.
Treatment With Antivirals
The standard treatment is the antiviral oseltamivir, taken twice daily for five days. This is the same class of medication used for seasonal flu, but the dosing schedule for bird flu is more aggressive. For seasonal flu prevention, a person typically takes one dose per day; for bird flu, treatment-level dosing (twice daily) is recommended even when the drug is given preventively after a high-risk exposure.
The key factor in treatment is speed. Antivirals work best when started as early as possible after symptoms appear. For people who are hospitalized with severe illness, treatment courses may be extended to 10 days. People who test positive for H5N1 but have no symptoms are still offered the five-day antiviral course.
Who Is Most at Risk
Bird flu is overwhelmingly an occupational hazard. Poultry farm workers, dairy farm workers, wildlife biologists handling wild birds, and anyone involved in culling infected flocks face the highest exposure risk. The recent U.S. cases reinforce this: nearly all 70 infections occurred in people with direct animal contact through their work.
For workers in high-exposure settings, the CDC recommends a specific set of protective equipment: a NIOSH-approved particulate respirator, fluid-resistant coveralls, safety goggles, boot covers, a head or hair cover, and disposable gloves. Optional additions include a waterproof apron and a face shield over goggles when there is risk of liquid splashing. Workers should avoid touching their face, and eating, drinking, or using phones while wearing contaminated gear. Showering at the end of a shift and changing into clean clothing before heading home are standard recommendations.
For the general public, the risk remains low. Avoiding contact with sick or dead wild birds, steering clear of raw (unpasteurized) milk from potentially affected herds, and cooking poultry and eggs thoroughly are the most practical precautions.
Vaccines for Bird Flu
No bird flu vaccine is currently available for routine public use, but preparation is well underway. The World Health Organization maintains a library of candidate vaccine viruses matched to different H5N1 strains, including the clade 2.3.4.4b viruses circulating in the current outbreak. These candidate viruses have passed safety testing and can be used by manufacturers to produce vaccines if the situation escalates. Several governments, including the United States, have stockpiled doses of older H5N1 vaccines and have contracts in place to scale up production rapidly if human-to-human transmission begins.
The challenge with bird flu vaccines is that the virus evolves, so a vaccine matched to one strain may not protect well against another. This is why WHO continuously updates its list of candidate viruses, similar to the annual process for seasonal flu vaccines. For now, the strategy relies on antiviral treatment, protective equipment for high-risk workers, and surveillance to catch any shift toward easier human-to-human spread.