Hong Kong (CNN) -- A third man in China has died from the H7N9 virus, a strain of avian flu not previously detected in humans, the Zhejiang provincial department of health said Wednesday, according to state-run media outlet Xinhua.
The disclosure of the third death comes only days after Chinese authorities announced the first three known cases of humans infected with the H7N9 bird flu virus on Sunday.
The total number of people infected with H7N9 in China has risen to nine, Xinhua reported Wednesday.
The death reported Wednesday was that of a 38-year-old man who passed away on March 27 in his home province of Zhejiang in eastern China, Xinhua reported. He worked in nearby Jiangsu province, where at least four other cases of humans infected with H7N9 were reported Tuesday.
Two other people who died -- men aged 27 and 87 -- lived in nearby Shanghai, according to Xinhua. The World Health Organization confirmed those deaths Monday.
Chinese authorities are trying to find the source of the human infections. They have so far said there are no signs of transmission of the H7N9 virus between any of the victims or people they have come into close contact with, suggesting the virus isn't highly contagious among humans.
They have also dismissed suggestions linking the infections with the discovery of thousands of pig carcasses from the Huangpu River which runs through Shanghai.
The Shanghai Animal Disease Prevention and Control Center on Monday tested 34 samples of pig carcasses pulled from the river and found no bird flu viruses, Xinhua reported.
On Tuesday, the Jiangsu provincial health bureau reported four cases of H7N9 in humans: a 45-year-old woman from Nanjing, a 48-year-old woman from Suqian, an 83-year-old man from Suzhou, and a 32-year-old woman from Wuxi.
The Nanjing woman worked culling poultry, it said.
Malik Peiris, a professor at Hong Kong University's School of Public Health, said Monday that the H7N9 strain of avian flu, already known to exist in wild birds, had probably been transmitted to poultry, and it infected the humans.
"It's really important to understand where this virus is coming from," he said.
Authorities in Shanghai are gathering daily data on cases of pneumonia resulting from unknown causes and will set up a team of experts to assess the "severity and risk" of H7N9, Xinhua reported Tuesday.
Since the transmission of these types of viruses 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.
The World Health Organization said Monday it was "in contact with the national authorities and is following the event closely."
Because there are so few cases of H7N9 detected so far, little research has been done, 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 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.
A spike in H5N1 deaths, many of them children, has been reported in Cambodia, prompting concern among health authorities.