The H7N9 influenza virus circulating in China has public health officials on edge.
It's the most lethal form of the virus they've seen in years, with one out of five infected patients dying from the illness.
Lab workers at the Centers for Disease Control's headquarters in Atlanta are working nonstop trying to develop a vaccine for H7N9.
So far those in China affected by the virus have had direct contact with infected but healthy-looking birds and there's no evidence the virus is spreading between humans, but the strain could mutate into a form that makes it contagious.
"If it develops the capacity to spread from one person to another, then it could cause very severe disease both in China and neighboring countries and around the world, including in the US," warns CDC Director Dr. Thomas Frieden.
It could take a year or more to develop a vaccine, and once it is available patients will likely need two shots to build immunity.
In the meantime, the CDC is alerting doctors to be on the lookout for cases among travelers so patients who test positive can be isolated immediately.
"There have already been two dozen travelers from China with severe illness in the US, we've tested them at CDC, all negative for this strain of flu," Dr. Frieden says.
Unlike past strains of bird flu H7N9 is not making birds sick, so it is hard to tell where the virus is spreading and which type of bird is infected.
If the virus does not mutate experts say H7N9 will likely not cause a pandemic.
Temporarily closing poultry markets, isolating infected patients and an active surveillance system could prevents its spread.