NEW: Authorities need to track down the source of the virus, an expert says "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. 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. 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.