BEIJING - Health officials say they still don't understand how a lesser-known bird-flu virus was able to kill two men and seriously sicken a woman in China, but that it's unlikely that it can spread easily among humans.

Two men in Shanghai became the first known human fatalities from the H7N9 bird-flu virus after contracting it in February. A woman in the eastern city of Chuzhou remains in serious condition, China's National Health and Family Planning Commission said.

It was unclear how the three patients became infected, the health agency said. It sought to calm fears about the virus but provided few details about each case. Authorities have not described the patients' occupations or said whether they had come into contact with birds or other animals.

The health authority noted, however, that two sons of one of the Shanghai men also suffered from acute pneumonia, one of whom died, and the source of their infection is still unknown. Other people who were in close contact with the victims have not become sick, indicating that the virus is not easily transmitted between humans.

"We don't know yet the causes of illness in the two sons, but naturally, if three people in one family acquire severe pneumonia in a short period of time, it raises a lot of concern," the World Health Organization's China representative, Michael O'Leary, said at a briefing in Beijing late Monday.

Many epidemiologists regard densely populated parts of China and Southeast Asia, where farmers often live in close quarters with pigs and poultry, as regions where conditions are ideal for nurturing infectious diseases that jump from animals to humans. An earlier deadly outbreak of SARS, or severe acute respiratory syndrome, was linked to wild animals that infected domestic animals, which in turn infected people in that region.

Other strains of the H7 family of bird-flu viruses have caused mostly mild human infections in the past, said University of Hong Kong microbiologist Malik Peiris. Cases were reported in the Netherlands, Canada, the U.S. and Britain - mostly following outbreaks in poultry.

Experts say the deaths in China might indicate that the H7N9 strain has morphed to become more lethal to humans, although it's not possible to make any conclusions yet about its mortality rate because many mild cases may go undetected. A thorough tracing of the virus is critical.

"I would guess that given the severity of the human disease it is likely that these particular viruses have undergone the change to become highly pathogenic, but obviously that remains to be ascertained," Peiris said. "The crucial question is the source of this virus: Where is it?"