A Chinese woman has died from an infection with H5N6 bird flu virus, the China News Service said Wednesday.

A second woman is in critical condition with the virus, which only very rarely infects people — but kills most of them when it does. Both cases were reported in China's southern Guangdong province.

Indian health workers wring the necks of chickens during a culling operation at Venkateshwara Hatcheries in Thoroor village, India on April 14. NOAH SEELAM / AFP - Getty Images

The H5N6 strain is one of several avian influenza viruses circulating in poultry in China and elsewhere and occasionally infecting people. It killed one woman last July, according to the World Health Organization, and two others in 2014 and 2015.

WHO says 683 laboratory-confirmed cases of H7N9 bird flu have been reported and the virus has killed 275 of those people. The H5N1 bird flu virus, circulating since 2003, has infected at least 844 people and killed 449 of them.

Related: CDC Watches H5N2 Bird Flu Virus

Scientists are keeping an eye on both to make sure they do not turn into strains that people can pass easily from one to another. That could cause a new pandemic of influenza if it happened.

There’s a milder virus occasionally infecting people, too, called H9N2. It’s only been detected in four patients so far. “This virus does not seem to transmit easily between humans and tends to result in mild clinical disease, therefore the current likelihood of community-level spread and public health impact of this virus is considered low,” WHO says.