Bird flu can affect humans in ways ranging from a mild eye infection to severe pneumonia and organ failure. Most people who catch it have direct contact with infected birds or, more recently, sick dairy cows. Since 2003, more than 23 countries have reported over 890 confirmed human cases of the H5N1 strain alone, and while the virus remains rare in people, it can be deadly when it does cross over.
Why Bird Flu Rarely Infects Humans
The reason bird flu doesn’t spread easily among people comes down to a mismatch in cell receptors. Avian influenza viruses latch onto a specific type of receptor found deep in the lungs and throughout the respiratory tracts of birds. Human influenza viruses, by contrast, prefer a different receptor type that lines the upper airways of people, like the nose and throat. Because the bird flu virus targets receptors that are sparse in the human upper airway, it has a hard time gaining a foothold.
This receptor difference is also why sustained person-to-person spread hasn’t occurred. Every past influenza pandemic, including 1918, 1957, and 1968, involved a virus that switched its preference to the human-type receptor. If H5N1 made that same switch, it could transmit between people far more efficiently, though it would also likely change in other ways that could affect its severity.
How People Get Infected
Nearly all human cases trace back to close contact with infected animals. Historically, that meant handling sick or dead poultry, visiting live bird markets, or working in environments where infected birds shed the virus in their droppings, saliva, or nasal secretions. Inhaling contaminated dust or touching contaminated surfaces and then touching your eyes, nose, or mouth are the primary routes.
In 2024, a new exposure source emerged in the United States: dairy cattle. Between April and September 2024, at least 14 human cases were reported, with four linked to sick dairy cows and nine to infected poultry during outbreak responses. Importantly, the CDC found no evidence of person-to-person transmission in any of these clusters. In one Missouri case where a household contact developed symptoms around the same time, investigators concluded the timing pointed to a shared exposure rather than one person infecting another.
Symptoms From Mild to Severe
The mildest form of bird flu in humans shows up as conjunctivitis, a redness and irritation of the eyes. All nine poultry workers infected during 2024 U.S. outbreaks had mild illness limited to eye symptoms. This pattern appears common among people with brief, lower-intensity exposures.
More serious cases look like a rapidly worsening respiratory illness. The Missouri patient, who had significant underlying health conditions, presented with chest pain, nausea, vomiting, diarrhea, and weakness. In severe H5N1 infections globally, complications can include pneumonia, respiratory failure, acute respiratory distress syndrome, kidney failure, multi-organ failure, sepsis, and inflammation of the brain (meningoencephalitis). The jump from mild symptoms to life-threatening illness can happen quickly, sometimes within days of symptom onset.
Why Severe Cases Turn Dangerous
One reason H5N1 can be so lethal is the body’s own immune response. In severe cases, the virus triggers a massive overproduction of inflammatory signaling molecules, sometimes called a “cytokine storm.” Researchers have found markedly elevated levels of these inflammatory signals in human cells and animal models infected with highly pathogenic H5N1. Under normal circumstances, these molecules help activate immune cells and fight infection. But when they spiral out of control, they cause excessive cell death and tissue damage, particularly in the lungs. This dysregulated response can be as destructive as the virus itself, flooding the lungs with fluid and impairing oxygen exchange.
Treatment With Antiviral Medications
Antiviral drugs are the primary treatment, and timing matters. The standard course for adults involves taking an antiviral twice daily for five days. Most circulating strains of H5N1, H5N6, and H7N9 respond to these medications, though they are often resistant to an older class of antivirals (adamantanes) that is no longer recommended.
Resistance to newer antivirals is uncommon but has been documented. Some hospitalized H5N1 patients have developed resistance during treatment, with fatal outcomes. Because of this risk, clinicians sometimes use a combination of two different antiviral drugs for hospitalized patients. If a patient’s respiratory symptoms worsen despite treatment, resistance is one of the first things medical teams consider. For people with mild illness, a single antiviral course is typically sufficient.
Vaccines and Preparedness
The U.S. licensed three H5N1 vaccines in 2007, 2013, and 2020, and additional vaccines have been authorized in Europe. These aren’t available at your local pharmacy the way seasonal flu shots are. Instead, the government maintains a national stockpile of vaccine components, including H5 antigen and adjuvants (ingredients that boost immune response), managed by a federal agency called BARDA. The idea is to have the building blocks ready so production can scale up fast if the virus begins spreading between people.
Newer vaccine technology is also in development. Arcturus Therapeutics launched a Phase 1 trial in December 2024 for a self-amplifying mRNA vaccine targeting H5N1. Moderna completed an early-stage trial for an mRNA vaccine candidate covering H5 and H7 strains, though a $590 million federal contract to accelerate that work was terminated as of May 2025. The WHO maintains and regularly updates a list of candidate vaccine viruses so manufacturers worldwide can prepare formulations that closely match whatever strain is circulating.
Current Risk to the General Public
For most people, the risk of catching bird flu remains very low. The virus does not spread efficiently between humans, and nearly every confirmed case involves someone who had direct, prolonged contact with infected animals. The people at highest risk are poultry workers, dairy farm workers handling infected cattle, and anyone involved in culling operations during outbreaks. Avoiding direct contact with sick or dead wild birds, wearing protective equipment when working with potentially infected animals, and practicing standard hygiene around livestock are the most effective precautions. The situation could change if the virus acquires the ability to bind human-type receptors more efficiently, which is why global surveillance remains intense.