Bird flu, or avian influenza, is an infectious disease that in most cases affects only birds. But a subtype of the virus called H5N1 can migrate to humans and is sometimes fatal.
H5N1 is one of several hundred subtypes of the Type A influenza virus and is not the only one to have infected humans. The H7N7 and H9N2 subtypes have also been found in humans, but it is a highly pathogenic strain of H5N1 that showed up in Asia in 1996 and caught the attention of public health authorities around the world.
Human cases of H5N1 "remain rare and sporadic events, occurring mostly in areas where H5N1 viruses circulate regularly in poultry," according to the World Health Organization (WHO). But a recent controversial case involving H5N1 research has again raised fears about the virus.
U.S. and Dutch scientists managed to mutate the H5N1 virus to make it more transmissible in ferrets, raising concerns that similar mutations could make the virus easily transmissible in humans.
The highly pathogenic H5N1 subtype was first isolated in farmed goose in Guangdong province in China and within a year it had made its way to poultry farms and live-animal markets in Hong Kong.
This strain distinguished itself from other bird flu viruses, which are common in wild ducks and geese but are mostly "low pathogenic," meaning they don't usually cause illness or kill large numbers, because it could jump from wild waterfowl to domesticated poultry and kill large numbers of birds immediately.
The first human infections of H5N1 occurred in 1997 in Hong Kong from the same strain of the virus that caused the bout of bird flu in Hong Kong poultry. Eighteen people developed severe respiratory disease caused by the virus, and six died.
Health officials determined that close contact with live infected poultry was the source of human infection in Hong Kong. It was also the first evidence that the H5N1 virus had been transmitted directly from birds to humans.
In the wake of the infections, officials ordered the destruction of Hong Kong's poultry population. More than 1.5 million birds were killed over three days.
Since then, more than 300 people have died from H5N1 and almost 600 are known to have been infected with the virus. According to the World Health Organization, human cases of H5N1 "remain rare and sporadic events, occurring mostly in areas where H5N1 viruses circulate regularly in poultry." ...
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