Last Friday, the Missouri health department announced a recent human case of bird flu. What’s unsettling about the case is that the patient — hospitalized on August 22 and later released — is the first among 15 people infected in the US who didn’t report having contact with animals. That raises the possibility that the illness has already begun spreading among humans.

It’s not yet clear whether the virus involved is the H5N1 influenza subtype that has infected wild birds, poultry, and dairy cows worldwide since it was first identified in 2020 and raised flags among experts about another potential pandemic. If it is, though, there’s concern about what the Missouri case could represent. “There’s a few steps before this potentially becomes a pandemic threat,” says Nahid Bhadelia, who directs the Boston University Center on Emerging Infectious Diseases. “But I’m a lot more worried about it than I was.”

Bird flu’s threat to most people remains pretty low. Still, here’s what makes this case concerning to experts, and what you can do to keep yourself safe.

Since the virus first spread to American dairy cows in January of this year, all 13 of the bird flu cases identified in humans before last Friday caused pretty mild disease — eye redness, otherwise known as conjunctivitis, and in one person, a cough without a fever.

That’s where the latest case is different: The patient was hospitalized, suggesting severe disease. The Missouri health department noted the adult patient “has underlying medical conditions,” but we don’t know their age or other risk factors.

For a flu virus to cause a human pandemic, says Seema Lakdawala, a virologist and flu expert at Emory University, it needs to overcome three hurdles: It must access and replicate efficiently within the respiratory tract to cause disease; it must spread easily from person to person; and it must be novel to our immune systems. If the virus infecting the latest case turns out to be H5N1, the fact the patient was hospitalized suggests this germ is evolving to replicate more efficiently in our airways (and coming closer to overcoming the first hurdle).

Another thing all previous US human H5N1 cases shared was that they could be traced back to close contact with infected farmed poultry or dairy herds. This case doesn’t appear to have that link.

Without a known exposure to an infected animal, there’s concern that H5N1 may be spreading in other ways — for example, through drinking raw milk or, even more consequentially, from person to person, which could lead to a much larger outbreak.

Some spread between household contacts or from a sick patient to health care workers has happened with H5N1 in the past, but hasn’t been sustainable, says Jennifer Nuzzo, an epidemiologist who leads the Pandemic Center at Brown University’s School of Public Health. However, spread through casual contact — for example, between people standing next to each other on a bus — hasn’t been reported with this virus, and would be much more worrisome.

Spread through casual contact would represent a step toward overcoming Lakdawala’s second hurdle for a pandemic: sustained person-to-person transmission. “The last time a flu virus did that, which was 2009, it was around the globe in a matter of days,” says Nuzzo.

It’s a promising sign that since the Missouri patient was hospitalized, there has not been a big bloom of flu-like illness in the state. “I don’t think there’s a whole iceberg out there that we don’t see,” says Nuzzo, but more details about the case would put her and other experts at ease.

Transmission through raw milk would also be bad news. Although most of the US milk supply is pasteurized — that is, heated to the point that bacteria and viruses are killed — about 1 in 100 Americans surveyed in the late 2010s said they drank raw milk every week. We don’t yet know whether the person who got infected in Missouri was a raw milk consumer, but that’s greatly preferable to the alternative of person-to-person spread, says Bhadelia. It’s more straightforward to control what you eat and drink than who you have contact with.

That long delay, says Nuzzo, suggests the US hasn’t switched on its sensitive flu surveillance system that would more rapidly identify bird flu infections in people with fever, cough, muscle aches, and other flu-like symptoms.

If there were a lot of person-to-person spread happening, having that system switched on would mean detecting that spread early — perhaps at a stage when it could still be contained. If the system isn’t on, however, transmission among humans could get out of hand before there’s a chance to stop it using the vaccines and medications that we know work to prevent severe flu symptoms and flu transmission.

The more infection there is among dairy cows, the more infection there’s likely to be among the people who work with them, and the more opportunities that are created for H5N1 to mutate into a virus that spreads efficiently between people.

Despite this risk, most states rely on farmers to self-report infections among their herds. Widespread distrust of government and public health across the agriculture industry means self-reporting is rare, and farms can’t be forced to test workers or animals unless their state agencies mandate it. “If these state public health agencies are not willing to do that, and the state governors are not willing to say, ‘Hey, there’s an ongoing outbreak that’s causing public health concern, we need to know what’s going on,’ we’re not going to have information,” says Lakdawala.

Although bird flu has been reported in Missouri only in poultry farms and not on dairy farms, that doesn’t mean it isn’t there, said Nuzzo. It just means farmers may not be testing their workers or cows for the virus, or they aren’t reporting it.

Many experts are concerned that Americans’ low trust in institutions would create enormous challenges in the event of another pandemic, especially one involving a virus transmitted through the air. That makes it particularly important to do what’s needed to get a handle on this outbreak, says Lakdawala. This would largely involve preventative measures instituted on dairy farms and having stockpiled vaccines available for those at highest risk of infection.

In a few months, the seasonal flu virus is going to be spreading in the US.

When animals get co-infected with different flu viruses at the same time, they can serve as “mixing vessels” for the viruses, allowing them to exchange segments of their genomes. That can lead to new viruses that are easier to transmit, better at causing disease, and above all, novel to the human immune system.

That could lead to getting H5N1 closer to jumping Lakdawala’s third hurdle. “We don’t want those two viruses to swap genes,” says Nuzzo.

There are steps you can take to minimize your already low risk.

Every episode of bird flu infection in a human makes a good case for a seasonal flu shot, says Nuzzo, especially in people who work on poultry or dairy farms. Every person who’s immunized against seasonal flu is less likely to be a mixing vessel for some new nightmare Frankenflu.

There’s a ton of H5N1 virus in raw milk: In April, 14 percent of American raw milk products contained the live virus, and the number may be higher now that more farms are affected. It’s never been safe to drink raw milk, but it’s particularly dangerous now; sticking with pasteurized dairy products is much safer, says Nuzzo.

Fall often brings farm festivals, and with them, opportunities for lots of contact with animals that might be asymptomatically infected with H5N1, says Lakdawala. “Don’t get close with animals,” Lakdawala says, and wash your hands often during and after a farm visit. If you work on a dairy farm or a veterinarian’s office, wear a face shield and other personal protective equipment (the Centers for Disease Control and Prevention (CDC) has some helpful recommendations) to protect yourself from splashes.

People tend to have only two settings when thinking about their risk of catching an emerging infection, says Bhadelia: “low risk and ‘oh my god.’” It’s more helpful to think of risk as a spectrum, and on that spectrum, Covid-19 and mpox currently pose a bigger threat to the general public than H5N1.

So if you’re someone who wants to avoid getting sick, Nuzzo suggests getting your Covid booster sometime this fall, wearing a mask in busy spots, and gathering outdoors instead of crowding inside. If you’d benefit from an mpox vaccine, get one.

For now, it’s mostly public officials’ and farm owners’ job to worry about bird flu. The best path forward for the general population is to control the variables you can, Nuzzo says. “Being worried is not a protective action.”