Bird flu outbreaks among dairy cows in multiple states, and at least one infection in farmworker in Texas, have incited fears that the virus may be the next infectious threat to people.

The influenza virus, called H5N1, is highly pathogenic, meaning it has the ability to cause severe disease and death. But while its spread among cows was unexpected, people can catch the virus only from close contact with infected animals, not from one another, federal officials said.

“It’s really about folks who are in environments where they may be interacting with cattle that are infected with this virus,” said Dr. Demetre Daskalakis, the director of the National Center for Immunization and Respiratory Diseases at the Centers for Disease Control and Prevention.

“The risk for most everyone else is very low,” he added. “Right now, our risk assessment hasn’t changed, but if it does change, we’re going be pretty quick and pretty transparent about that.”

Avian influenza is often fatal in birds, but none of the infected cows have died so far. The only symptom in the patient in Texas was conjunctivitis, or pink eye, which was also reported in people infected during other bird flu outbreaks.

The C.D.C. and other agencies in the United States and elsewhere have tracked H5N1 for years to monitor its evolution. Federal agencies have stockpiled vaccines and drugs to be used in a possible bird flu outbreak.

“We are more ready for an influenza pandemic than probably any other outbreak that could occur, any other pathogen,” said Rick Bright, the chief executive of Bright Global Health, a consulting company that focuses on improving responses to public health emergencies.

Dr. Bright led influenza preparedness at the Biomedical Advanced Research and Development Authority, or BARDA, the federal agency that supports research on vaccines and drugs for emergencies, for several years before he served as the agency’s director from 2016 to 2020.

Here’s what you need to know about the H5N1 virus:

Is a human pandemic inevitable?

Among birds and animals, the H5N1 bird flu is already a pandemic, or a panzootic, with infections observed on every continent except Australia. To date, the virus has not evolved into a form that can spread easily from one person to another, and it may never do so.

As its name indicates, H5N1 has primarily been a problem in birds. But it has now spread to a wide range of species, from sea birds and small scavengers like foxes to large mammals, like bears and cows.

There have been sporadic infections in people since 1997, when a cluster of cases appeared in Hong Kong. But most patients worldwide have been in very close contact with infected animals, and generally they did not pass the virus to other people.

To become adept at transmission between people, H5N1 would need to pick up several additional mutations and shift its shape. The strain that was isolated from the infected farmworker in Texas carries one of those mutations, but that change has appeared before — in people, foxes and seals, among others — without further consequences.

The infections in people so far “fortunately are all still single-time cross-species transmission,” said Vincent Munster, a virologist at the National Institute of Allergy and Infectious Diseases who has studied the mutations needed for H5N1 to adapt to people.

History suggests that even if the virus changes enough to begin widespread transmission between people, it may have to give something up in return, Dr. Munster said. For example, when other flu viruses have adapted to humans, they have lost much of their virulence, causing only mild symptoms.

How will we know if the virus spreads to people?

H5N1 is an influenza virus that is followed by extensive scientific networks that track flu viruses worldwide.

“We’ve got our eye on it, and we’ve had our eye on it for years,” Dr. Daskalakis said.

These surveillance networks tracked H5N1 even before its surge in birds and animals over the past two years. Now they are on high alert. Scientists are watching for mutations that could make H5N1 more likely to infect people or resistant to the vaccines and drugs available to counter it.

The World Health Organization, the C.D.C. and other global health organizations routinely share information and genetic sequences to monitor which flu strains are spreading and where.

In the current outbreak, the Department of Agriculture has shared genetic sequences from infected cows with the C.D.C., which analyzes the sequences and ensures that the stockpiled vaccines and drugs are still effective.

Do we have a vaccine for bird flu?

Yes.

BARDA has enough building blocks for vaccines — including adjuvants, substances that can enhance a vaccine’s strength — to make millions of doses in weeks. Mass production could also ramp up quickly if needed, federal officials said.

The C.D.C. already has two candidate viruses that can be used to make vaccines. As the virus changes — gaining mutations that make it resistant to the current vaccines and drugs, for example — federal researchers may create newer candidates.

Three pharmaceutical companies can be called on to make vaccines for bird flu, but those vaccines would be manufactured on the same production lines that are used to make seasonal flu vaccines. Before embarking on large-scale manufacturing, federal officials would have to consider the implications of disrupting seasonal production, said David Boucher, the director of infectious disease preparedness and response at the Department of Health and Human Services.

The pharmaceutical companies don’t all use egg-based methods to produce vaccines, an important consideration because of the potential for bird flu outbreaks to derail the nation’s egg supply. BARDA is also looking to add mRNA to the list of technologies that can be used to make bird flu vaccines. (The Covid-19 vaccines made by Pfizer and Moderna relied on the method.)

What about treatments?

At least four antiviral medications are available to treat people who may become sick with bird flu, including the widely available generic drug oseltamivir, sometimes marketed as Tamiflu.

Unlike the vaccines, which are stockpiled by the federal government, the antiviral drugs are available commercially. Generic versions of oseltamivir are made by many manufacturers worldwide.

The federal government has a stockpile of tens of millions of doses of oseltamivir, Dr. Boucher said. The government is in close communication with manufacturers that could quickly ramp up the production of oseltamivir, as it has in the past during some bad flu seasons.

All of these preparations are in place for a worst-case scenario, but “we’re not there yet,” Dr. Boucher said. “Our job here is to prepare for the worst and get ready for it in case it does come.”