A Louisiana patient who had been hospitalized with severe bird flu has died, the first such fatality in the United States, state health officials reported on Monday.

The patient was older than 65 and had underlying medical conditions, the officials said. The individual became infected with the bird flu virus, H5N1, after exposure to a backyard flock and wild birds.

There is no sign that the virus is spreading from person to person anywhere in the country, and Louisiana officials have not identified any other cases in the state. Pasteurized dairy products remain safe to consume.

“I still think the risk remains low,” said Dr. Diego Diel, a virologist at Cornell University.

“However, it is important that people remain vigilant and avoid contact with sick animals, sick poultry, sick dairy cattle, and also avoid contact with wild birds,” he added.

The news comes on the heels of a report that the patient had carried mutations that might help the virus infect people more easily.

The Centers for Disease Control and Prevention said late last month that the mutations were not present in virus samples taken from the backyard flock, suggesting that they developed in the patient as the illness progressed.

One of the mutations was also present in virus sampled from a 13-year-old Canadian girl who was hospitalized and required respiratory support. She has since recovered.

Both patients carried a version of the virus that is circulating in wild birds, distinct from the one causing the outbreak in dairy cattle.

Although these are isolated cases, the two together point to the potential for the virus to morph into dangerous new forms, experts have said.

The news “should remind us that H5N1 influenza has been and continues to be a dangerous virus,” said Dr. James Lawler, a director of the University of Nebraska’s Global Center for Health Security.

“The more widely the virus circulates, particularly infections in humans and other mammals, the higher the risk that the virus will acquire mutations that adapt the virus for human disease and transmission,” he said. “This puts us all at risk.”

That risk is particularly heightened as the nation confronts a severe flu season.

An individual who is simultaneously infected with both the bird flu virus and the seasonal flu might provide H5N1 ample opportunity to acquire the mutations it needs to spread efficiently among people.

H5N1 has been circulating in wild birds for several years and in dairy cattle for about a year. The outbreak has shown no signs of abating, affecting more than 900 herds in 16 states. The virus has also spread from dairy farms to poultry farms, and remains widespread in wild birds.

In December, California, the state hit hardest by the outbreak in cattle, declared a public health emergency.

At least 66 people have been infected by the virus in the United States this year, according to the C.D.C. Nearly all of the cases have been in people who worked on farms with infected cows or poultry.

Most people have had mild symptoms, often conjunctivitis, or pink eye, and respiratory symptoms. Globally, there have been about 500 deaths reported in the past 20 years, most of them in Southeast Asia.

The Louisiana patient was reported to have been hospitalized last month. But state officials have declined to release further details, citing patient confidentiality.

Before last year, just one human H5N1 infection — in a poultry worker in Colorado in 2022 — had been reported in the United States.

Experts have warned against drinking raw milk, which may contain high levels of the virus. No human cases have yet been linked to raw milk, but cats in multiple states have died after drinking virus-laden milk.