California has declared a state of emergency over the outbreak of bird flu among dairy cows.

H5N1, the virus that causes bird flu, or avian influenza, has been found in 645 dairies in California since August. Infected herds have also been identified in 15 other states. Here's what we know, and don't know, about the risk to human health right now.

Gov. Gavin Newsom of California said in a statement that the declaration was "a targeted action to ensure government agencies have the resources and flexibility they need to respond quickly to this outbreak."

The Centers for Disease Control and Prevention reiterated Wednesday that H5N1 bird flu poses a low risk to the public's health overall. And Michael Osterholm, an infectious disease expert at the University of Minnesota, agreed with that assessment. The main concern in California, he said, is the rapid spread of the virus among dairy farms.

Still, some people are at higher risk of infection because they are exposed to animals that spread the virus. People who work closely with animals, like dairy farmworkers, are particularly susceptible and should wear personal protective equipment such as masks, gloves and safety goggles, said Stacey L. Schultz-Cherry, a virus and influenza expert at St. Jude Children's Research Hospital.

There have been 61 human cases of bird flu reported in the United States so far this year, including 37 cases linked to exposure to infected cattle. The majority of these patients have had mild disease, with symptoms including pink eye, fever and muscle aches.

On Wednesday, the CDC confirmed the first severe case of H5N1 bird flu in the country: a patient in Louisiana, who was hospitalized after being exposed to backyard flocks. The patient was infected with a version of the virus that has been found in birds, not the one that is spreading in cows.

If you have a backyard flock, try to limit their contact with wild birds, said Dr. Meghan Davis, a veterinarian and environmental epidemiologist at the Johns Hopkins Bloomberg School of Public Health. She also stressed the importance of keeping pets, particularly cats, away from wild birds, as some cats have become infected with bird flu and died.

"If you're not working directly with animals right now, it would not be the top thing that I'd be worried about," said Dr. Gail Hansen, a veterinary public health expert and consultant based in Washington, D.C. "It's unlikely right now that's going to be a problem for most people."

There isn't any evidence so far that the virus can spread among humans. Every time the virus infects another person or animal, however, it has an opportunity to mutate, and scientists are closely watching whether the virus will gain mutations that make it more easily able to spread from human to human.

"I've been sleeping with one eye open about this virus for many years," Osterholm said. It's important to track the virus and be prepared for the possibility that it could spread among humans, he said.

Researchers are particularly concerned about the risks of the virus mutating during flu season, and encouraged people to get their flu shots to reduce the risk of contracting seasonal influenza.

"The biggest fear for any of us is that you get the wrong person, bird, animal, whatever, infected with a human seasonal and one of those bird strains, and those viruses are able to do what they do best, which is genetically swap components," Schultz-Cherry said.

Health authorities have stressed that pasteurized milk is safe to drink. But officials are worried about the risks of raw milk -- especially in California, where two raw-milk producers have recalled their products after bird flu was detected in retail samples.

The U.S. Department of Agriculture has ramped up its testing of raw milk, hoping to better identify the scale of the outbreak.

No human cases have been confirmed from drinking raw milk. But scientists say they think farmworkers may have contracted bird flu after coming into contact with droplets of raw milk.

"This could be a problem -- don't completely write it off," said Matthew Moore, an associate professor in the department of food science at the University of Massachusetts, Amherst. "But right now, there's no reason to panic or really do anything beyond avoid raw milk products."