The new bird flu in China has come with a long list of questions.
Are the 82 cases reported so far just the tip of a larger outbreak? Why does the virus cause mild symptoms in some people and severe pneumonia in others?
Perhaps the most critical question is also the simplest: How do people catch the bug?
The H7N9 virus clearly infects birds. Health workers have detected it in chickens, ducks and pigeons.
But many people who have gotten sick didn't came near birds, the World Health Organization said Wednesday.
A scientist at the China Disease Prevention and Control Centre said that nearly 40 percent of patients with H7N9 don't have a clear history of poultry exposure, The Beijing News reported Tuesday.
Yet there's still little evidence that the virus can easily jump between people.
"We've seen four instances that could be human-to-human transmission among very close contacts," WHO spokesman Gregory Hartl tells Shots. "Even in these cases, the transmission could be from a common environmental source. If it was human-to-human transmission, it wasn't sustained."
So what's fueling the outbreak?
"It might be other animals. It might be that the case histories they have aren't complete enough. It could be human-to-human transmission," Hartl says.
It may even be dust at market. Health workers have detected the virus in "samples from the environment" in several Shanghai markets, the World Organization for Animal Health reported in early April.
The group identified 11 H7N9 outbreaks in birds around China. But H7N9 doesn't make birds sick. So detecting the virus in animals has been like finding a needle in a China-sized haystack.
"Something like 47,000 to 48,000 birds have been tested, and only 39 have come back positive, " Hartl says. "Those come from poultry, duck and pigeons."
Most of the human H7N9 cases have occurred in Shanghai or the surrounding region. But there have been two cases in Beijing, which is nearly 800 miles away from Shanghai.
Most people infected have been hospitalized with severe respiratory problems and 17 have died. But Hartl says there have been a handful of mild cases. "At least one person wasn't hospitalized." And, one boy in Beijing tested positive for the virus but showed no symptoms at all.
Nailing down where the virus comes from and how it moves around seems to be at the top of the WHO's to-do list. It has a team currently headed to China to help the government do just that, Hartl says.
"If we want to control this virus," he says, "we need to know where it lives and how it's transmitted."