At least one person in Texas has been diagnosed with bird flu after contact with dairy cows that were presumed infected, state officials said Monday.

The announcement adds a worrying dimension to an outbreak that has affected millions of birds and marine mammals around the world and, more recently, cows in the United States.

So far, there are no signs that the virus has evolved in a way that helps it spread more easily among people, federal officials said.

The patient’s main symptom was conjunctivitis; The individual is being treated with an antiviral medication 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 cows in New Mexico and Idaho may also be infected.

The virus has been identified as the same version of H5N1, a subtype of influenza, that circulates among birds in North America.

The CDC is working with state health departments to monitor other people who may have come into contact with infected birds and animals, the agency said Monday.

This is only the second case of H5N1 bird flu in people in the United States; the first was in 2022. Experts said 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 USDA said in its announcement last week.

Here’s what you should know:

What is bird flu?

Avian influenza, or avian influenza, is a group of influenza viruses that primarily adapt 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 rapidly around the world. In the United States, it has affected more than 82 million farm birds, the worst bird flu outbreak in U.S. history.

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

Experts say H5N1 does not yet appear to have adapted to spread efficiently between people.

How did cows get bird flu?

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

“The fact that they are susceptible — the virus can replicate and make them sick — 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 to emerge in Texas and New Mexico. Dead birds were also found on some of these farms and laboratory tests eventually confirmed that the cows were infected with bird flu.

There are a variety of ways the virus could have reached livestock. 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 to dairy farms.

How have the cows been affected?

Although the virus is usually fatal in birds, it appears to cause a relatively mild illness in cows.

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

The disease mainly affects older cows, which have developed symptoms including loss of appetite, mild fever and a significant drop in milk production. The milk cows produce is often “thick and discolored,” according to Texas officials. The virus has also been found in unpasteurized milk samples collected from sick cows.

Experts warned that it is still unclear whether the bird flu virus is the sole cause of all the symptoms and illnesses that have been reported.

How widespread is the problem?

It is not clear. As of last Friday, the USDA’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 the national laboratory has not yet confirmed those findings. Until now it has only been found in dairy cows and not in 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 livestock between states could transport the virus to new places. The affected dairy in Michigan had recently imported cows from one of the infected Texas herds. At the time of transportation of the cows, the animals did not present any symptoms. The farm in Idaho had also recently imported cows from an affected state, Idaho officials said.

How is it spreading?

This is a key question and still unanswered. It is possible for all infected cows to contract the virus independently, especially if shared food or water sources have been contaminated.

However, a more worrying possibility is that the virus is spreading from one cow to another. On Friday, the USDA noted that “transmission between livestock cannot be ruled out.”

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

If it turns out that the virus spreads 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 will acquire mutations that make it more dangerous to people.

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

Analysis of 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 monitoring infections in birds and marine mammals and, now, cows. So far, the virus does not appear to have acquired the ability to spread efficiently between people.

In 2012, scientists showed that H5N1 could spread through the air between ferrets (a popular model for studying the transmission of respiratory viruses between people) after acquiring five mutations.

An avian flu sample isolated from a Chilean man last year had two mutations that indicate adaptation to infect mammals. But such mutations have been observed before without the virus evolving further and spreading between people, experts said.