A fast-growing outbreak of avian flu has upended California’s dairy industry, the nation’s largest producer of milk, infecting most of the state’s herds and putting thousands of farmworkers at risk for contracting the virus.

In just about four months, cows in 645 dairies in California have tested positive for H5N1, even as many ranchers have taken strict precautions to stop the virus from spreading. Gov. Gavin Newsom was concerned enough Wednesday that he declared a state of emergency over the outbreak in California.

The virus is spreading so quickly that dairy farmers are calling it “Covid for cows,” and scientists are racing to figure out how to stop the contagion.

“We’re trying to do everything we possibly can, and this has just been the worst crisis we’ve ever dealt with in the dairy industry in California,” said Anja Raudabaugh, the chief executive of Western United Dairies, a trade organization that represents most of the state’s dairy farms.

Avian flu primarily affects birds, but it can also infect mammals, including humans. There have been 61 human cases reported in the United States so far this year, and most of the individuals have had mild symptoms, including pink eye, fever and muscle ache.

But officials reported on Wednesday that the nation’s first severe human case of infection had been identified in an individual in Louisiana who had been hospitalized with bird flu.