A new bird flu strain that has sickened more than 100 and killed 22 in China is "one of the most lethal" of its kind, and has now spread to another country, according to the World Health Organization (WHO).
A 53-year-old Taiwan businessman contracted the H7N9 strain of bird flu while travelling in China, Taiwan's Health Department said Wednesday. This is the first reported case outside China's mainland.
The man was hospitalized after becoming ill three days after returning from Suzhou on April 9, Health Department Minister Wen-Ta Chiu told a news conference. Chiu said the patient was diagnosed with the H7N9 virus and was in serious condition.
Chiu said Taiwan will take appropriate measures, including opening a special out-patient clinic for H7N9 cases.
The Centers of Disease Control and Prevention is keeping a close eye on this virus, which may be resistant to some antibiotic drugs.
The CDC received another sample of the virus from China this week, and Mike Shaw, who works at the CDC's flu lab, said they found certain mutations of the virus, which had they mutated further, would have been rendered ineffective against antivirals, like Tamiful and Relenza.
Dr. Joseph Bresee, a flu expert at the CDC, said it is possible the flu is being transmitted from person to person, but it's not yet been 100 percent confirmed.
When two or three members of a household become sick with the same virus, it's hard to know if they all acquired it from an infected bird they were exposed to, or if they passed the virus between them, Bresee said.
However, at this point, it's not clear how exactly people are being infected. Experts maintain there is no evidence of sustained transmission between people. An international team of scientists led by the WHO and Chinese government spent five days in China investigating - - but were not able to determine if the virus was actually spreading between people.
The CDC is racing to produce a vaccine for this new strain of bird flu, but it's still several months out, and officials said it's difficult to determine if the vaccine would even work.
"The situation remains complex and difficult and evolving," said Keiji Fukuda, the WHO's assistant director-general for health security. "When we look at influenza viruses, this is an unusually dangerous virus for humans."
Fukuda said that although the current outbreak has a lower fatality rate than the previous type of bird flu, "this is one of the most lethal influenza viruses that we've seen so far."
Scientists who have analyzed genetic sequence data from samples from three H7N9 victims say the strain is a so-called "triple reassortant" virus with a mixture of genes from three other flu strains found in birds in Asia.
Recent pandemic viruses, including the H1N1 "swine flu" of 2009/2010, have been mixtures of mammal and bird flu -- hybrids that are more likely to be milder because mammalian flu tends to make people less severely ill than bird flu.
Pure bird flu strains, such as the new H7N9 strain and the H5N1 flu, which has killed about 371 of 622 the people it has infected since 2003, are generally more deadly for people.
The team of experts, who began their investigation in China last week, said one problem in tracking H7N9 is the absence of visible illness in poultry.
Fukuda stressed the team is still at the beginning of its investigation, and said that "we may just be seeing the most serious infections" at this point.
Based on the evidence, "this virus is more easily transmissible from poultry to humans than H5N1", he said.
Besides the initial cases of H7N9 in and around Shanghai, others have been detected in Beijing and five provinces.
Samples from chickens, ducks and pigeons from poultry markets have tested positive for H7N9, but those from migratory birds have not, suggesting that "the likely source of infection is poultry," said Nancy Cox, director of the influenza division at the U.S. Centers for Disease Control and Prevention.
John Oxford, a flu virologist at Queen Mary University of London, said the emergence of human H7N9 infections -- a completely new strain in people -- was "very, very unsettling."
"This virus seems to have been quietly spreading in chickens without anyone knowing about it," he told Reuters in London.
Flu experts say it is likely that more cases of human infection with H7N9 flu will emerge in the coming weeks and months, at least until the source of infection has been completely confirmed and effectively controlled.
The WHO's China representative, Michael O'Leary, issued figures last week showing that half of the patients analyzed had no known contact with poultry.
Reuters and Fox News' John Roberts contributed to this report.