The H5N1 avian flu, commonly known as bird flu, has infected more than 100 million birds in the US and nearly 500 dairy herds in 15 states. The virus has emerged in mammals including elephant seals, goats, foxes and domestic cats.

Despite its widespread spread among animals, federal health authorities say the risk to the American public remains low. There have been only 46 confirmed human cases in the US during the current outbreak. All but one of the people had been exposed to affected poultry or livestock, according to the U.S. Centers for Disease Control and Prevention (CDC), and all of their illnesses were mild. The CDC says there is currently no evidence that the virus is spreading from person to person.

However, recent news is making some people uncomfortable. On November 12, Canadian health authorities announced that an otherwise healthy teenager who contracted bird flu from an unknown source is in critical condition and struggling to breathe, underscoring the potential seriousness of the disease – and its sometimes mysterious spread.

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In October, the US Department of Agriculture also sounded the alarm when it announced that a pig in Oregon had tested positive. That’s a worrying development because pigs can be infected with swine, human and bird flu viruses, making them excellent “mixing vessels,” said Meghan Davis, associate professor of environmental health and engineering at the Johns Hopkins Bloomberg School of Public Health. If a pig were infected with multiple strains of flu at the same time, the viruses could potentially combine to form a new strain that could spread widely among people, Davis explains. That’s what happened during the 2009 “swine flu” pandemic.

“There are a lot of things I worry about,” Davis says. “This is one of them.”

Another cause for concern: Flu season is underway in the U.S., and as the seasonal flu virus makes its rounds, “people can be a mixing vessel themselves,” Davis says. If a person were infected with bird flu and seasonal flu at the same time, the two viruses could theoretically combine to create a more transmissible variant.

The good news is that so far there is no evidence that the avian virus has undergone enough changes to easily infect and spread among humans, says Troy Sutton, assistant professor of veterinary and biomedical sciences at Penn State University.

Why did some people get sick in the first place, if the virus is not good at infecting people? At a high enough dose — if a farm worker is in close contact with sick animals, for example — the virus can sometimes enter human cells, even though it isn’t built to do exactly that, Sutton says. But, crucially for public health, the virus does not appear to have evolved in a way that makes it easy for those who get sick to pass the disease on to others.

That appears to be partly because the virus cannot grow well in the human nose, Sutton says. The seasonal flu is highly contagious because it takes root in the upper respiratory tract. When a sick person coughs, sneezes, or even talks, they can release infectious respiratory droplets. The bird flu virus is not as common in the upper respiratory tract, so it appears to be less transmissible to humans, Sutton says.

However, this does not mean that spread through the respiratory tract is impossible. Two recent studies in ferrets — one by researchers at the CDC and one led by a University of Wisconsin-Madison researcher — have raised that possibility. The researchers isolated the bird flu strain that sickened the first person in the current outbreak and tested how contagious it was among ferrets. Although not as contagious as seasonal flu, the bird flu virus could spread among ferrets through droplets, the researchers found.

But there are important caveats, Sutton says. Ferrets – even though they are often used in flu research – are not a perfect parallel for humans. And the strain studied is similar, but not identical, to the strain currently spreading widely among cows. Overall, the CDC concluded, “The virus remains unable to spread efficiently among humans via respiratory droplets compared to seasonal flu viruses.”

Still, health authorities are preparing in case the situation changes. In October, the US Administration for Strategic Preparedness and Response (ASPR) gave $72 million to pharmaceutical companies that make H5 flu vaccines, telling them to use the money to prepare shots “should they be needed now or in the future.” according to the US government. a statement from an agency. ASPR representatives said they were taking this step out of an “abundance of caution.” Federal health officials have not recommended that anyone get vaccinated against H5N1, and vaccinations are not publicly available.

Getting a regular seasonal flu shot won’t protect against bird flu, but it’s still a good idea to get one, especially for people who work or regularly come into contact with animals. The CDC also recommends that farmworkers wear personal protective equipment, such as masks and goggles, and take flu antivirals as soon as possible in case of exposure.

But there is not much to do for the general public at the moment, health authorities say. Just don’t drink raw milk (since the virus can persist without pasteurization), avoid touching dead or sick animals, and get your seasonal flu shot.