Avian influenza, commonly known as bird flu, is an infectious disease caused by influenza A viruses that primarily infect birds. Avian influenza viruses are classified according to their characteristics and the species they infect. Most avian influenza viruses do not infect humans, but some viruses can occasionally cross the species barrier and cause human infections.

Human infection with avian influenza was first recognized in Hong Kong in 1997, when highly pathogenic H5N1 viruses infected humans. Since then, H5N1 and other avian influenza viruses have caused sporadic human infections in different parts of the world. Beginning in 2020, a major global spread of highly pathogenic H5N1 among wild birds and poultry also resulted in infections in a growing number of mammals. Human infections remain uncommon, but people who have close or prolonged contact with infected birds or mammals can be at increased risk.

Avian influenza viruses can infect many species of birds and some mammals, including poultry and wild birds, as well as mammals such as foxes, skunks, seals, and other animals. Wild birds can carry and spread some avian influenza viruses, sometimes with few or no obvious symptoms, while highly pathogenic strains can cause severe disease and death in poultry and some wild birds. Viruses can spread among animals through respiratory secretions, saliva, feces, and contaminated environments.

People can become infected primarily through close contact with infected animals or contaminated environments. Human-to-human transmission of currently circulating avian influenza viruses is considered uncommon, although influenza viruses can change and therefore continue to be closely monitored. Properly cooked poultry and eggs are considered safe to eat; cooking poultry and eggs thoroughly also destroys influenza viruses and other harmful microorganisms.

Symptoms of avian influenza in humans can range from no symptoms or mild illness to severe disease. Symptoms may include fever, cough, sore throat, muscle aches, conjunctivitis, and shortness of breath. Severe infections can cause pneumonia, respiratory failure, or other serious complications. Anyone who develops symptoms after close contact with infected birds or other infected animals should seek medical advice and report the relevant exposure.

By Michael Chiu, MWS, CI
Previously published in Ming Pao Daily News – Updated and Revised in 2026