International health experts are gathering in China to begin a week-long investigation into how the H7N9 virus is spreading. The virus has already killed 17 people and sickened 70 others. When Chinese health officials reported the virus almost three weeks ago, they called it bird flu. But new cases suggest that the virus is spreading to people who have not had any contact with fowl.

“To me, the biggest question is the link between the virus in birds and how it gets to humans. This is not clear,” Dr. Bai Chunxue, a respiratory expert in Shanghai, told The Associated Press. ”So this is indeed a mystery.”

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One of the families Chunxue treated has said they had no contact with birds or poultry. In fact, as many as 40 percent of patients have reported no exposure to poultry or other birds.

The rapid spread of the virus, and its death rate of 20 percent, sparked the country to invite the World Health Organization and the U.S. Centers for Disease Control and Prevention to bring experts to Beijing and Shanghai to parse out where the virus originates. There is  concern that the virus may originate in other animals or environmental sources, and that the virus could mutate and spread from humans to other humans.

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The World Health Organization is already examining three Shanghai families and two Beijing boys to see whether they may have infected each other, spokesman Gregory Hartl told The New York Times.

Because patients usually don’t succumb to death for a couple of weeks after infection, the mortality rate may rise, director of the Center for Infectious Disease Research and Policy at the Univesity of Minnesota Michael Osterholm told USA Today.

Photo: A woman wears a face mask inside a subway station in Shanghai, on April 5, 2013. Corbis