A worrying development in China’s H7N9 outbreak: There is growing evidence that the virus may have the ability to be transmitted between humans, especially close family members, and the Chinese government has admitted for the first time it’s a possibility.

The Chinese National Health and Family Planning Commission said on Thursday it could not rule out human-to-human transmission in the case of a Shanghai family—two brothers, at least one of whom has the virus, and their 87-year-old father, who was the first confirmed H7N9 fatality. A husband and wife in Shanghai also both contracted H7N9.

“Further investigations are still under way to figure out whether the family cluster involved human-to-human transmission,” said Feng Zijian, director of the health emergency center of the Chinese Center for Disease Control and Prevention, according to the state-run China Daily. “Human-to-human transmission, in theory, is possible, but is highly sporadic.”

Some of the H7N9 patients have had no contact with poultry, making human-to-human transmission a real possibility, the World Health Organization said on Wednesday. There have been 82 cases of H7N9, with 17 fatalities.

There is a crucial difference between a virus capable of limited human-to-human transmission, during long periods of close contact between family members, and effective human-to-human transmission, as with seasonal flu that can spread with incidental contact.

The H1N1 flu, a virus found in pigs and birds that caused several hundred thousand deaths in 2009 and 2010, was capable of limited human-to-human transmission. A strain of avian flu with a high mortality rate that is capable of effective human-to-human transmission, on the other hand, would have the makings of a disaster film.