At least one person in Texas has been diagnosed with bird flu following contact with dairy cows presumed to be infected, state officials said on Monday.

The announcement adds a worrying dimension to an outbreak that has affected millions of birds and sea mammals worldwide and, most recently, cows in the United States.

So far, there are no signs that the virus has evolved in ways that would help it spread more easily among people, federal officials have said.

The patient’s primary symptom was conjunctivitis; the individual is being treated with an antiviral drug and is recovering, according to the Centers for Disease Control and Prevention.

The Department of Agriculture announced the first cases in dairy herds in Texas and Kansas last week, and then, a few days later, in an additional herd in Michigan. Preliminary testing suggests that cows in New Mexico and Idaho may also be infected.

The virus has been identified as the same version of H5N1, an influenza subtype, that is circulating in North American birds.

The C.D.C. is working with state health departments to monitor others who may have been in contact with infected birds and animals, the agency said on Monday.

This is only the second case of H5N1 bird flu in people in the United States; the first was in 2022. Experts said that they believe the risk to the general public remains low. But testing and analysis is ongoing, and there are many unanswered questions.

“This is a rapidly evolving situation,” the U.S.D.A. said in its announcement last week.

Here’s what to know:

What is bird flu?

Bird flu, or avian influenza, is a group of flu viruses that are primarily adapted to birds. The particular virus in these new cases, called H5N1, was first identified in 1996 in geese in China, and in people in Hong Kong in 1997.

In 2020, a new, highly pathogenic form of H5N1 emerged in Europe and spread quickly around the world. In the United States, it has affected more than 82 million farmed birds, the worst bird flu outbreak in U.S. history.

Since the virus was first identified, there have also been sporadic cases in people in other countries. In 2023, there were 248 cases of people infected with H5N1 virus, and 139 deaths, according to the World Health Organization. But a vast majority resulted from prolonged, direct contact with birds.

H5N1 does not yet seem to have adapted to spread efficiently among people, experts say.

How did cows get bird flu?

Cows were not thought to be a species at high risk.

“The fact that they are susceptible — the virus can replicate, can make them sick — that is something I wouldn’t have predicted,” said Richard Webby, an influenza virologist at St. Jude Children’s Research Hospital.

But earlier this year, reports of sick cows began emerging in Texas and New Mexico. Dead birds were also found on some of these farms, and laboratory testing eventually confirmed the cows were infected with bird flu.

There are a variety of ways that the virus might have found its way into cattle. The most likely route, several experts said, is that infected wild birds, which shed the virus in their feces and oral secretions, contaminated the cows’ food or water.

But other free-ranging animals known to be susceptible to the virus, such as cats and raccoons, could also have brought the virus onto dairy farms.

How have cows been affected?

Although the virus is often fatal in birds, it appears to be causing relatively mild illness in cows.

“It’s not killing animals, and they seem to be recovering,” said Dr. Joe Armstrong, a veterinarian and cattle production expert at the University of Minnesota Extension. Last week, the U.S.D.A. said that there were no plans to “depopulate,” or kill, affected herds, which is the standard procedure when poultry flocks are infected with the virus.

The disease is primarily affecting older cows, which have developed symptoms that include a loss of appetite, a low-grade fever and a significant drop in milk production. The milk that the cows do produce is often “thick and discolored,” according to Texas officials. The virus has also been found in unpasteurized milk samples collected from sick cows.

It is not yet clear whether the bird flu virus is the sole cause of all of the symptoms and illnesses that have been reported, experts cautioned.

How widespread is the problem?

It’s unclear. As of last Friday, the U.S.D.A.’s National Veterinary Services Laboratory had confirmed bird flu infections in two herds in Texas, two herds in Kansas and one herd in Michigan. Initial testing has suggested that additional herds in Texas, New Mexico and Idaho may also have the virus, but those findings have not yet been confirmed by the national laboratory. So far, it has been found only in dairy cows and not in beef cattle.

But because cows are not routinely tested for bird flu, and the illness has been relatively mild, there could be other infected herds that have escaped detection, experts said.

And the movement of cattle between states could transport the virus to new locations. The affected dairy in Michigan had recently imported cows from one of the infected Texas herds. At the time the cows were transported, the animals were not displaying any symptoms. The farm in Idaho had also recently imported cows from an affected state, Idaho officials said.

How is it spreading?

That is a key, and still unanswered, question. It is possible that the infected cows are all picking up the virus independently, especially if shared food or water sources have been contaminated.

A more worrisome possibility, however, is that the virus is spreading from cow to cow. On Friday, the U.S.D.A. noted that “transmission between cattle cannot be ruled out.”

Several scientists said that they would be surprised if there was not some degree of cow-to-cow transmission. “How else could it move so rapidly?” said Dr. Gregory Gray, an infectious disease epidemiologist at the University of Texas Medical Branch.

If the virus turns out to spread easily between cows, that could lead to larger, more sustained outbreaks. It would also give the virus more opportunities to adapt to its new mammalian hosts, increasing the risk that it acquires mutations that make it more dangerous to people.

How will officials know if bird flu adapts to spread between people?

Analyzing the genetic sequence of the virus from infected birds, cows and people can reveal whether H5N1 has acquired mutations that help it spread among people.

Scientists have been closely tracking infections in birds and sea mammals and, now, cows. So far, the virus does not seem to have gained the ability to spread efficiently between people.

In 2012, scientists showed that H5N1 was able to spread through air between ferrets — a popular model for studying transmission of respiratory viruses among people — after acquiring five mutations.

A sample of bird flu isolated from a Chilean man last year had two mutations that indicate adaptation to infecting mammals. But those mutations have previously been seen without the virus evolving further to spread between people, experts said.