Story highlights Authorities need to track down the source of the virus, an expert says

A woman remains in a critical condition after contracting the virus

Chinese authorities say it hasn't been found in humans before

They say it doesn't appear to be highly contagious among humans

Two people in China have died and another remains critical after falling ill with a strain of bird flu not detected before in humans, the official Chinese news agency Xinhua reported.

Both of those who died, men aged 27 and 87, lived in Shanghai, while a 35-year-old woman in Chuzhou city in nearby Anhui province is in the hospital, the Chinese National Health and Family Planning Commission said Sunday, according to Xinhua.

The victims all had initial symptoms of fever and coughing that then developed into severe pneumonia and difficulty in breathing, Xinhua reported.

A team of experts assigned by the health commission established that the three cases were human infections of H7N9 avian influenza, which has not been found in humans previously, the news agency reported.

Investigators haven't yet figured out how the three people contracted the virus, according to the commission. It ruled out the theory that they had infected one another.

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"It's really important to understand where this virus is coming from," said Malik Peiris, a professor at Hong Kong University's School of Public Health.

He said the H7N9 strain of avian flu, already known to exist in wild birds, had probably been transmitted to poultry, from where it infected the humans.

None of the 88 people who had close contact with the victims have shown symptoms of the flu, the commission said, suggesting the virus isn't highly contagious among humans.

According to Xinhua, the 87-year-old man fell ill on February 19 and died on March 4; the 27-year-old man became unwell on February 27 and died on March 10; and the female victim got sick on March 9.

The Chinese Center for Disease Control and Preservation on Friday separated the H7N9 bird flu virus from samples from the victims, Xinhua reported.

Finding the source

Since the transmission of these types of virus from animals to humans is usually "extremely inefficient," there are often tens of thousands of infected birds for every human case, according to Peiris.

As a result, "it is very likely that there is a quite widespread outbreak happening" among the animals from which it came, he said, underscoring the urgent need to track down the source.

Because the three people are the only human cases of H7N9 detected so far, little research has been carried out on it, according to Xinhua. There are no known vaccines against this virus, it said.

But Peiris said it was likely that existing anti-flu drugs, such as Tamiflu, are likely to work against the H7N9 strain. He also noted that the World Health Organization (WHO) has identified the H7 virus family as a potential threat and earmarked possible vaccine candidates.

He said other strains from the H7 family had caused previous outbreaks in poultry in countries including the Netherlands, Britain, Canada, the United States and Mexico. Human infection was documented in all of those cases except the Mexican one.

The outbreak of the H7N7 strain in the Netherlands in 2003 infected 89 people, one of whom died, according to Peiris.

The better known H5N1 avian flu virus has infected more than 600 people since 2003, of which 371 have died, according to the WHO.

In February, China reported two new human cases of H5N1 in the southern province of Guizhou, both of whom were in a critical condition, the WHO said.