(CNN) -- Six more cases of human infection with H7N9 bird flu have been reported in China, according to the World Health Organization. One of them was fatal, it said Friday in a news release.
WHO cited reports it received Wednesday from the National Health and Family Planning Commission (NHFPC) of China, and the Centre for Health Protection (CHP).
All of those infected were males, ages 2 to 75 years, and four were in critical or serious condition. Four of the six were reported to have been exposed to poultry or to a live poultry market.
The cases are from Fujian (1), Guangdong (1), Hong Kong (1) and Zhejiang (3). The fatality, a 75-year-old man who had traveled late last month to Shenzhen, where he became ill, died in Hong Kong, but did not appear to have been infected there, WHO said.
"So far, there is no evidence of sustained human-to-human transmission," WHO said.
WHO recommends that travelers to countries with outbreaks of bird flu stay away from poultry farms and avoid contact with animals in live bird markets, entering slaughterhouses for poultry and contact with feces.
But the organization does not recommend any travel or trade restrictions.
From February to May last year, 133 human H7N9 cases were reported in a first wave, followed by just two reported cases in July and August and, since last October, 74 more in a second wave, WHO said.
Health authorities have strengthened H7N9 prevention and control measures, including the closure of live poultry markets in some affected cities.
This virus had not been seen in animals or people until February 2013, according to WHO.
Since then, infections in people and birds have been identified, with most of the people becoming severely ill, and most of them reporting recent exposure to live poultry or possibly contaminated environments.
As of January 28, 22% of people known to have been infected had died; two thirds of those infected were male, with a median age of 58 years.
The disease is marked by rapidly progressing pneumonia, fever, cough and shortness of breath.
Treatment typically includes administration of anti-flu drugs oseltamivir (brand name: Tamiflu) and zanamivir (brand name Relenza). Vaccines have not been shown to be effective.
There are many different strains of avian flu: 16 H subtypes and 9 N subtypes. Only those labeled H5, H7 and H10 have caused deaths in humans.