Clinical research has found that nearly 60 percent of the more than 100 people infected with the H7N9 virus are aged 60 or older.

Meanwhile, the number of infected male patients is double that of females, said Zhong Nanshan, head of Guangdong province's expert committee on the prevention and control of H7N9 bird flu.

Seniors, and those with chronic conditions such as hypertension, diabetes, cerebrovascular, chronic obstructive pulmonary and coronary diseases are most easily infected. The gender difference, however, is not statistically significant, according to Zhong in a phone interview on Friday.

The 76-year-old expert in respiratory diseases played a leading role in the fight against Severe Acute Respiratory Syndrome in 2003.

By Thursday, there were 128 confirmed cases of H7N9 bird flu on the Chinese mainland, resulting in 26 deaths, according to the World Health Organization.

"For now, the best way to prevent H7N9 infection is to avoid contact with live poultry and to stop slaughtering birds in food markets," said Zhong.

He added that medical experts have found no direct evidence to prove that people can avoid H7N9 by wearing masks.

However, for those already infected, the chances of the illness becoming more serious decrease significantly if drugs such as Tamiflu are taken within five days of the viral attack, said Zhong at a meeting of the Guangdong provincial government on Thursday.

Both central and local governments are strengthening the monitoring of live poultry, and culls have been undertaken in areas where carriers were discovered. The measure has been effective in controlling the disease in Shanghai.

By Friday, four "family clusters", where two or three members were infected with the virus, had been reported. Although a number of reasons for this have been put forward, experts cannot completely rule out the possibility of human-to-human transmission, said Zhong.

"In these families, different members were infected with H7N9 at different times. For some families, the time lag is pretty long; for others, pretty short. Therefore, it's possible that they were infected by the same sources, rather than by each other," he said.

Among the 128 confirmed cases, 106 were reported in Shanghai and the provinces of Jiangsu and Zhejiang, including 25 deaths. These areas may have a comparatively high number of poultry carrying the virus, said Zhong, although he ruled out a link with the large number of dead pigs found floating in the Yangtze River in March, because the gene fragments of the H7N9 virus are of avian origin.

"Compared with its response to SARS 10 years ago, the government has taken swifter measures to prevent and control H7N9, and greatly improved the transparency of information. The government has made huge steps in terms of handling live poultry carrying H7N9 by closing the relevant markets, employing early detection and treatment and by centralized treatment of severely infected patients," he said.