Shares of four North American airlines that fly to China, including Air Canada, fell on Tuesday as fear grew about th virus that authorities say has killed six people and sickened 300 more. Investors seemed to worry that the virus could spread beyond Asia, like the SARS outbreak that started in 2002. Chinese authorities also confirmed that the virus can spread from one person to another, not just from animals to humans.

In afternoon trading, shares of Air Canada fell by nearly five percent, United Airlines’ parent fell three percent, Delta lost more than two percent, and American Airlines was off less than one percent.

The outbreak is believed to have started in Wuhan in central China. None of the US airlines fly to Wuhan, but their Chinese partner airlines do, and some passengers do transfer from a Chinese carrier to an American one.

Fear about the virus increased just as millions of Chinese prepared to travel for the lunar New Year holiday, which starts Saturday. Cases have also been confirmed in Japan, South Korea and Thailand, and now in the US.