Two more people were diagnosed with bird flu this week, even as scientists in Missouri continued to investigate a possible cluster of infections in that state, federal health officials said at a news briefing Friday.

In California, two farmworkers who were exposed to infected dairy cattle at different farms tested positive for the virus, called H5N1, state health officials said Thursday. Those cases bring the total this year to 16, not including those under investigation.

The cases do not come as a surprise, because the number of infected herds in California has risen to 56 from 16 two weeks ago, said Dr. Nirav Shah, the principal deputy director at the Centers for Disease Control and Prevention.

“As there are more herds that test positive, there are more workers who are exposed, and where there are more workers who are exposed, the chances of human infection increase,” he said. The risk to the public remains low, he added.

Still, experts said that the appearance of H5N1 in multiple states was worrisome.

Flu viruses are adept at acquiring new abilities by swapping their genes. As the flu season swings in, even one person who becomes infected with both bird flu and the seasonal flu virus could help H5N1 to gain the ability to spread as readily among people as seasonal flu does.

Given the many variables, it’s difficult to gauge the true risk of the virus mutating into a more contagious form, said Jesse Bloom, an evolutionary biologist at the Fred Hutchinson Cancer Center in Seattle.

“The answer could be not at all, or we could have a major human outbreak,” he said. “Both of those are possible.”

Last month, the CDC announced that a patient in Missouri with no known exposure to infected animals had been hospitalized with gastrointestinal symptoms and had tested positive for H5N1. Last week, the agency said that one household member and six health care workers who had been in contact with that patient had all developed symptoms.

CDC officials said Friday that they did not know whether the symptomatic individuals in Missouri had been tested for other illnesses, and they referred those queries to state health officials.

The workers developed symptoms before the initial patient was diagnosed with H5N1, according to Missouri officials. The single worker who sought testing at that time was negative for multiple respiratory viruses, including flu.

Another worker who had persistent symptoms and sought care long after the exposure window was negative for COVID, mononucleosis and strep.

It is possible that the workers had COVID-19 or some other illness with flulike symptoms. A summer COVID wave is just beginning to abate; although it did not overwhelm hospitals, many people became ill.

Because the hospitalized patient did not carry high amounts of virus when tested, officials have said that it is unlikely that the others were infected with H5N1.

The chances of human transmission or a cluster of illnesses may well be low, said Angela Rasmussen, a virologist at the University of Saskatchewan in Canada.

“But it’s hard to say that with a lot of confidence,” she said, “when you haven’t done the test to rule it out.”

“It seems to me that there’s a big motivation to avoid panic, or avoid fear, and avoid criticism,” she added, referring to the official response.

The CDC and Missouri health officials have repeatedly said that the hospitalized patient and the household contact did not have “typical” symptoms of seasonal flu or H5N1, but rather gastrointestinal symptoms, including vomiting and diarrhea, making a true H5N1 infection unlikely.

In its external communications, the CDC has cited only conjunctivitis, mild flulike upper respiratory symptoms, fever and muscle aches as possible symptoms of bird flu infection. The agency did not mention gastrointestinal problems.