Hong Kong (CNN) -- Another case of bird flu has been reported in China, taking the total infection count to 83 people, as health authorities inside and outside the country try to determine how to stop its spread.
The most recent case was detected in a 38-year-old poultry trader in Henan province, according to state-run Chinese news agency Xinhua.
Earlier, the World Health Organisation said an additional 19 cases were found in the eastern provinces of Zhejiang and Jiangsu as well as the city of Shanghai.
Seventeen people have died from the H7N9 strain of the virus which, while common in birds, hadn't been detected in humans before the first cases were reported in March.
The WHO says it's still exploring the possibility whether the virus can be spread between people.
"In a significant percentage of cases there is no known contact with poultry. It means that we still don't know what the disease reservoir is," WHO spokesman Timothy O'Leary told CNN.
"We need to establish where the virus is living in which animal it has its reservoir and then try to figure out how it's being transmitted to people. All the evidence points to animal-to-human transmission but which animal and how is it being transmitted? This is the big mystery."
Authorities are monitoring more than 1,000 people who have come into close contact with confirmed cases, but O'Leary stressed "so far there's no evidence of ongoing or sustained human-to-human transition of the virus."
Particular attention is being paid to clusters of people who have contracted the illness, including one family where a father and two sons fell ill.
The 87-year-old father died of the virus in March, followed soon after by his younger son. It hasn't been confirmed whether the son had the illness, according to Feng Zijian, director of the health emergency center of the Chinese Center for Disease Control and Prevention. Because he had died, further tests weren't possible, Feng added.
The elder son was confirmed to have had the virus but has since recovered. He is one of five people who have been discharged from hospital after treatment, according to Xinhua.
So far, one boy is confirmed to have been an asymptomatic carrier of the virus, meaning he tested positive for the illness but didn't display any symptoms. The discovery of an asymptomatic carrier is worrying because it could make the spread of the infection more difficult to monitor, experts say.
The four-year old was part of a sweep of people tested for the illness because they'd come into close contact with the first reported case in Beijing.
Authorities took throat swabs from a group of people connected to 24 poultry farmers in Naidong Village, Cuigezhuang County in Beijing's Chaoyang District, according to the Beijing Municipal Health Bureau.
Meanwhile, a number of international experts have arrived in China, at the invitation of the Chinese government, to help local health authorities trace the source of the illness, the WHO said.
The team includes Chinese experts on epidemiology, laboratory tests and clinical treatment, as well as experts and officials from the U.S, the European Union and the WHO, who will "assess the outbreak and the response to guide further the prevention and control measures," WHO spokesman Glenn Thomas told reporters in Geneva on Tuesday.
Journalist Peter Shadbolt in Hong Kong contributed to this report.