What is H5N1 bird flu, and what has happened since 1997?

H5N1 is a type of influenza A virus that causes avian influenza, usually called bird flu. It is common in many bird populations and spreads quickly through poultry flocks and wild birds. It can also infect mammals, including humans, that have been exposed to infected birds, and in those cases the symptoms can be severe or fatal. Human cases are rare, though.

The story of H5N1 in people began in 1997. In Hong Kong that year, 18 people who had been in close contact with poultry were infected and 6 of them died, the first known cases of H5N1 in humans. The territory culled 1.3 million chickens. After 2003, human cases kept appearing. Between 2003 and December 2025 the World Health Organization recorded 993 confirmed cases and 477 deaths. The real fatality rate is presumably lower, because mild cases may never have been identified as H5N1.

What keeps scientists watching is what has not happened. As of February 2024 there had been very few instances of human-to-human transmission, and each outbreak had been limited to a few people. The virus is built for birds: it prefers to bind to a different kind of receptor than human flu viruses do. But since 2018 the highly pathogenic form has become the dominant strain in wild birds worldwide, and it has been recorded in mammals including cattle, seals, goats and skunks.