Bird flu has been hitting a little too close to home lately. In its testing of the commercial milk supply, the U.S. Food and Drug Administration (FDA) reported on April 25 that 20% of milk samples tested from the retail market contained “viral fragments” of H5N1 bird flu. Many believe that’s an underestimate; experts at Ohio State University have found that as much as 40% of milk samples from processing facilities in the Midwest may contain parts of the virus.

The results immediately raised concerns about the safety of the U.S.’ milk supply and the risk of infection for people consuming it. So far, the U.S. Centers for Disease Control and Prevention (CDC) and World Health Organization say that the risk remains low for people to get infected from milk. In the U.S., there have only been two known recent cases of H5N1 in humans: one in a poultry worker in 2022, and another in March in a person who worked with dairy cows.

The situation is changing quickly. But here’s what we know right now about the risks of bird flu to the milk supply.

Is milk safe to drink?

The FDA says its tests found fragments of the virus—which doesn’t mean that the viruses were live and able to infect and cause disease. The agency’s scientists are conducting additional tests to determine whether the fragments are still infectious, which would help them decide if drinking affected milk can lead to infection. “Pasteurization is effective in inactivating HPAI [Highly Pathogenic Avian Influenza],” the FDA said on its website. So far, “testing did not detect any live, infectious virus. These results reaffirm our assessment that the commercial milk supply is safe.”

Milk on store shelves is pasteurized, which generally kills viruses, and farmers have taken steps to discard milk from sick cows, the FDA says.

Read More: Is It Safe to Eat Eggs and Chicken During the Bird Flu Outbreak?

On April 24, the U.S. Department of Agriculture (USDA) issued a federal order requiring that all cattle be tested for the virus responsible for the current outbreak—H5N1 clade 2.3.4.4b—before moving from one state to another, in order to prevent spread to other herds or facilities. The agency is also asking—but not requiring—that farmers submit milk from lactating cattle and nasal swabs from non-lactating cattle who become sick to the National Animal Health Laboratory Network for testing. And any state labs and veterinarians who find positive tests must report them to the USDA’s Animal and Plant Health Inspection Service.

But even those steps may not be able to stop the spread of the virus, since it’s not clear how many cows may be infected and asymptomatic (and therefore untested). So far, it appears that the avian flu is milder in cows than in birds, in whom it can be fatal. “We’ve heard reports of cows with essentially no signs of disease that are testing positive,” says Andrew Bowman, associate professor of veterinary preventive medicine at Ohio State University. (Bowman was the scientist who conducted his own test of retail milk and found that 40% of samples had viral fragments.) “It’s one of those things that we are going to have wrap our heads around; we can’t just be reliant upon clinical signs to identify infected animals.”

Has H5N1 been found in beef?

Health officials are still investigating whether the virus is in beef, but Alcaine says so far it looks like the virus is mostly found in the mammary glands of cows. “We’re still trying to understand how cow-to-cow transmission is happening,” he says. “But it’s not really shed in the feces, and it looks like the viral loads are lower in the nasal cavity than in the mammary glands.” The same appears to be true for cow muscle. And it’s not clear whether both male and female cows can be infected, since most of the testing so far has been in milk-producing females.

How widespread is bird flu in cattle?

As of April 26, the USDA confirmed 34 bird flu outbreaks in cattle in nine states. However, testing is relatively sparse, compared to the number of cattle in the U.S.

Why are health officials worried about the milk supply?

While the milk supply is currently thought to be safe, things could change quickly, health experts say.

They’re watching how the virus moves from species to species and what genetic changes it picks up as it makes these jumps. Bird flu strains aren’t generally adept at infecting other species, including mammals. But the most recent case of bird flu in a person was also the first time the virus has been found in cows.

The fact that it’s now infecting cows—animals that people come in closer contact with than other mammals that have harbored H5N1, like foxes—means the viruses could potentially be mutating in ways that could spread and cause disease in significantly more people.

Bowman says that the FDA’s report is concerning because it suggests that this particular strain of H5N1 is continuing to be transmitted among cows. “This is a spillover into a mammalian host that seems to be maintaining [the infection],” he says. “In previous spillovers into mammals, it seemed to be for the most part individual events that were isolated and didn’t continue to spread in those species. This is different.”

“Every time another animal or human is infected, it’s another throw at the genetic roulette table in terms of whether the virus could become one that transmits from human to human, which is what is required for a pandemic,” says Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota. “If you throw enough times, you may end up with an outcome that you don’t want.”

Read More: Experts Can’t Agree If We’re Still in a Pandemic

The biggest worry is if avian influenza strains start popping up in pigs. Pigs tend to be an effective vector for viruses from different species, which pigs then pass on to people, since their cells share traits in common with other animals and with humans. If that happens, then it’s more likely that a human-compatible version of bird flu might jump to people.

On farms, that scenario wouldn’t be a stretch, since cows and pigs often coexist there. The USDA issued dairy workers guidance to increase efforts to clean milking equipment, spilled milk, clothing, vehicles, and other animals that may come in contact with milk. The agency has also warned that infected, unpasteurized milk could be a source of spread to other animals and potentially even people.

More testing is necessary to determine if there is any risk of the virus spreading via airborne particles that are spewed from an infected animals. “The risk is not just respiratory from a breathing cow, but could be in aerosols created in the milking process itself,” says Osterholm.

That’s why the USDA is also recommending that dairy workers use personal protective equipment, such as masks and other body coverings, to limit their exposure to any viral particles.

For now, at least, the danger to people remains low to the general public, Alcaine says. “The [infected] cows are recovering and look like they are going back to producing milk just fine,” he says. Still, “it will take some time to understand how it is impacting the dairy cow population.”