Why Is the Latest 'Most Lethal' Bird Flu Outbreak Worse Than All the Others?

What began as a bird flu strain not before seen in humans that killed four at the beginning of this month is now responsible for the deaths of more than 20 people.

The new strain H7N9 is being called one of the "most lethal" flu viruses so far, worrying health officials because it can jump more easily from birds to humans than the one that started killing people a decade ago, World Health Organization officials said.

Scientists are watching the virus closely to see if it could spark a global pandemic but say there is little evidence so far that it can spread easily from human to human.

WHO's top influenza expert, Dr. Keiji Fukuda, told reporters at a briefing in Beijing that people seem to catch the H7N9 virus from birds more easily than the H5N1 strain that began ravaging poultry across Asia in 2003. The H5N1 strain has since killed 360 people worldwide, mostly after contact with infected fowl.

Here are pictures showing that residents are taking the spread of the virus seriously:

Health experts are concerned about H7N9's ability to jump to humans, and about the strain's capacity to infect birds without causing noticeable symptoms, which makes it difficult to monitor its spread.

"This is definitely one of the most lethal influenza viruses we have seen so far," Fukuda said. But he added that experts are still trying to understand the virus, and that there might be a large number of mild infections that are going undetected.

Watch WHO's report:

The H7N9 bird flu virus has infected more than 100 people in China, seriously sickening most of them and killing more than 20, mostly near the eastern coast around Shanghai. Taiwan on Wednesday confirmed its first case, a 53-year-old man who became sick after returning from a visit to the eastern Chinese province of Jiangsu.

Will the virus make its why to the U.S.? According to a health expert in an ABC report, yes:

"Given the extent of global travel, I expect that we will see cases in the United States," ABC News chief health and medical editor Dr. Richard Besser said. "It's so important that people who become ill tell their doctors if they have been traveling."

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U.S. health officials are also bracing for bird flu by preparing a vaccine, a process expected to take six months. In the meantime, they, too, are urging travelers to China to steer clear of birds, practice good hygiene and report any and all flu-like symptoms to a doctor upon return.

"The CDC has developed a diagnostic test for H7N9 flu so that travelers who develop symptoms can be tested," Dr. Besser said.

In comparison, the earlier bird flu strain, H5N1, is known to kill up to 60 of every 100 people it infects.

Wednesday's briefing came at the end of a weeklong joint investigation by WHO and Chinese authorities in Beijing and Shanghai.

Experts said they still aren't sure how people are getting infected but said evidence points to infections at live poultry markets, particularly through ducks and chickens. They said it was encouraging that reported infections appeared to slow down after the closure of live poultry markets in affected areas.

The Associated Press contributed to this report.