Bird flu is spreading among wild birds, chickens and turkeys across much of the United States but health authorities said Wednesday there is a low risk of people getting sick.

Since December 2014, cases of the highly pathogenic H5 avian influenza virus have spread through 16 states in the midwestern and western parts of the country, as well as some parts of Canada, infecting nearly seven million birds, according to the US Department of Agriculture.

The H5 virus in North America looks different than the H5N1 bird flu circulating in Asia, where it has jumped from birds to people, said Alicia Fry, branch medical officer at the US Centers for Disease Control and Prevention.

The CDC considers the risk to the general US public to be "low at this time," she told reporters.

"That said, human infections with similar avian influenza viruses have occurred and it is possible that we may see human infections with the viruses associated with recent US bird flu outbreaks."

Most cases of human infection with bird flu have occurred when people had prolonged and close contact with infected birds.

"While we are cautiously optimistic that there will not be human cases, we must be prepared for that possibility," she added.

Health authorities are continuing to study the virus and are working on a potential vaccine that people could take.

However, the arrival of warmer weather in spring and summer could stop the virus in its tracks, at least for now.

"We know that this virus doesn't like heat so when it gets up to a certain level of temperature this virus doesn't survive, usually," said John Clifford, the chief veterinary officer of the US Department of Agriculture.