Avian influenza viruses (also known as avian flu and bird flu) are influenza A viruses that are generally found in the intestines of wild birds. Migratory wild birds, asymptomatic carriers of the virus, can infect domestic birds such as chickens, ducks, and turkeys. Infection makes the domestic birds sick, and most die. Sporadic infections in mammals with highly pathogenic avian influenza (HPAI) A(H5N1) virus have been reported in the United States, Canada, and other countries, and multidirectional interspecies mammalian transmissions have been documented.

Humans acquire avian influenza viruses primarily through direct contact of the mucous membranes with infectious secretions and excreta from infected birds or contaminated poultry products. The major portal of entry appears to be the upper respiratory tract. Although human-to-human transmission has been suggested in several household clusters, so far, there has been no sustained human-to-human transmission. Much remains to be learned about the exact mode of transmission. Most human illness from avian influenza has resulted from infection with Asian lineage H7N9 and H5N1 viruses, although there have been rare reports of human illness from H5N6, H5N2, H10N3, H9N2, and H3N8 strains.

Illness in humans from all bird flu virus infections is rare and has ranged in severity from no symptoms or mild illness to severe disease that resulted in death. Continued sporadic human infections are anticipated. In 2023 and 2024, there were several cases of human infection with HPAI A(H5N1) in Cambodia, including some deaths; all patients reportedly had a history of recent exposure to sick or dead poultry prior to their illness, with no indication of person-to-person spread.

Human cases of H5N1 linked to outbreak among US dairy cows and poultry: HPAI in dairy cows was first reported in Texas and Kansas by the US Department of Agriculture on March 25, 2024, and has since been identified in herds across multiple states. As of December 3, 2024, there have been 57 human cases, with all but two reporting exposure to infected dairy cattle or poultry. There has been no evidence of person-to-person spread. The risk to the general public remains low, but unpasteurized (raw) milk consumption should be avoided.

In November 2024, a human case of HPAI was reported in Canada; the infection was from a strain of the H5N1 virus circulating in wild birds.

Traditionally, the incubation period is generally between 2-5 days, but an upper limit of 8 days is possible. Most patients have headache, malaise, high fever, sore throat, cough, shortness of breath, and myalgia. Conjunctivitis, watery diarrhea, abdominal pain, vomiting, pleuritic pain, and bleeding from the nose have also been reported. Respiratory distress, tachypnea, and inspiratory crackles are present on physical examination. Lymphopenia and thrombocytopenia are commonly present. The frequency of milder illnesses, subclinical infections, and atypical presentations such as encephalopathy is not known. Pregnant individuals are at increased risk for severe illness from influenza.

Atypical presentations of avian influenza have been reported. Patients have developed nausea, vomiting, and diarrhea preceding acute respiratory failure. Progression to acute respiratory distress syndrome (ARDS) and respiratory failure is common. Complications have included bacterial sepsis, pulmonary hemorrhage, and multi-organ failure. The mortality rate of hospitalized patients has been high due to progressive respiratory failure.