On September 6, the Missouri health department announced it had recently identified a new human case of bird flu. This case was different from the other 15 human cases previously identified in the US in that the patient denied having contact with animals, raising the possibility that the illness had already begun spreading among humans.

On September 13, the Centers for Disease Control and Prevention (CDC) quietly released new information about the case: There were two more possible bird flu infections in people who’d had close contact with the first patient. According to a CDC website update, one was a household contact of the patient who got sick with similar symptoms on the same day as the case; they weren’t tested and have since recovered. The second case was in a health care worker who developed mild symptoms and tested negative for flu.

The CDC said in a press briefing only the day before, on September 12, that to their knowledge, none of the patient’s close household contacts had gotten sick. The confusing messaging has led to disquiet about the lack of transparency in US public health communications.

It was unclear why the CDC did not share the information initially: The Missouri health department has not requested the CDC’s assistance with its investigation of the case, and states typically determine what patient information they share with the CDC during outbreaks.

In its press briefing, CDC officials did reveal that while the virus’s subtype would likely not be identified, genetic analyses showed it is “closely related” to the H5N1 influenza subtype circulating among dairy cows, which experts fear could lead to another pandemic. “There’s a few steps before this potentially becomes a pandemic threat,” said Nahid Bhadelia, who directs the Boston University Center on Emerging Infectious Diseases, in early September. “But I’m a lot more worried about it than I was.”

CDC officials also said the initially identified patient had “significant underlying conditions” and that the patient’s symptoms — which included chest pain, nausea, vomiting, diarrhea and weakness — were more typical of a stomach bug than of classic influenza infection.

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 initial Missouri 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. The additional cases reported on September 13 fall into these categories. 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. In the September 12 briefing, CDC principal deputy director Nirav Shah said the patient had not reported raw dairy consumption.

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 even longer delay preceding the announcement about the other two cases creates additional concern, and Nuzzo said in a follow-up email that the lack of detail surrounding those cases is also worrisome. “I am deeply concerned that the household contact was not tested … [and] that health authorities have not disclosed the dates on which the hospitalized patient or the health care worker were tested,” she wrote. “These data are standard reported data in outbreaks and are essential for interpreting the assessment that this virus was not transmitted between people.”

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 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.”

Update, September 17, 5:55 pm ET: This story was originally published on September 10 and has been updated to include information about newly identified cases and additional comment from a source.