Bird Flu: 8 Fast Facts You Should Know

As of December 2024, an outbreak of the H5N1 virus, also known as bird flu or avian influenza, has spread from California and infected 58 people in seven states. Almost all of them are farm workers who, according to the Centers for Disease Control and Prevention, likely caught it through contact with infected dairy cows. Here what you need to know about the disease and its possible risks to the public.

1. The virus been found in more than 100 bird species.
2. Human infections are rare—but often serious. 
3. Humans are usually infected through close contact with domestic birds.
4. Bird flu is making eggs more expensive.
5. H5N1 was recently found in dairy cow herds—and scientists are alarmed. 
6. Pasteurized milk is safe to drink.
7. Raw milk can carry the virus.
8. Scientists are concerned about the virus mutating into something more dangerous to people. 

The virus been found in more than 100 bird species.

H5N1 is respiratory virus that primarily infects birds. It’s one of a group of viruses that cause highly pathogenic avian influenza (HPAI), which has been found in more than 100 avian species. Wild waterfowl such as ducks, geese, swans, and gulls seem to be frequent carriers and they pass it on to farm birds, like chickens and turkeys. Outbreaks are most common in the spring and fall when birds migrate. Wild waterfowl are much less likely to get sick or die from H5N1 than domesticated birds. 

Human infections are rare—but often serious. 

Bird flu has likely been endemic for centuries. In 1878, Italian parasitologist Edoardo Perroncito first described a “fowl plague.” In the decades after, farmers and naturalists marked several outbreaks, including mass sickness and death in New York City’s live bird markets in the mid-1920s. By the 1950s, the phenomenon had been documented on five continents, and in 1959, scientists isolated the first virus responsible for highly pathogenic avian influenza: H5N1. 

Avian influenza was not considered a threat to humans until 1997, when an outbreak in Hong Kong coincided with 18 people becoming sick with severe flu-like symptoms. Six of them died. The strain was unique from past H5N1 viruses, indicating that “bird” flu had made a jump to humans.

Infections are still very rare among people, but the fatality rate is more than 50 percent. Since 1997, 948 people have been known to be infected with H5N1 and 464 of them died. 

Humans are usually infected through close contact with domestic birds.

Infected birds shed avian influenza viruses through saliva, mucus, and feces. Of the relatively few people who have become sick with H5N1, most came into contact with this material through work on poultry farms. 

Transmission of H5N1 from human to human has been documented, but it’s very rare and scientists think the disease does not easily transmit that way. Public health authorities say you cannot get bird flu from eating properly prepared eggs and poultry [PDF]. 

Bird flu is making eggs more expensive.

Occasional H5N1 outbreaks have been a fact of life in the poultry industry, but in the last three years, the problem has gotten a lot worse. A particularly devastating U.S. outbreak began in February 2022. The CDC estimates that more than 108 million birds in 48 states were infected. In October 2024, as the disease lingered and rebounded, more than 2.8 million birds died. 

The bird flu deaths are definitively a factor in the recent increase in egg prices, which rose from an average of $1.84 a dozen in September 2021 to $3.82 in September 2024.

H5N1 was recently found in dairy cow herds—and scientists are alarmed. 

In March 2024, investigators discovered dairy cows in the U.S. were infected with H5N1. For those who study the disease, this was a wake-up call. Bird flu is rare in cows, and it had never before infected entire herds. “There’s the occasional cow infected, but they are not a natural host for influenza A viruses,” University of Colorado immunologist Jenna Guthmiller said, “and so this is really quite shocking to the field.” 

The virus has since spread to more than 700 herds in the U.S. Some farms in California and Colorado have been placed under quarantine and some states are requiring testing for cows acquired from out of state.

Cows are much larger than birds and shed more biological material that could carry H5N1. The process in which they are packed into warehouse-like facilities and milked with mechanical equipment is causing concern that infectious effluvia could be passed around a herd quickly.

Pasteurized milk is safe to drink.

To be extra cautious, the U.S. Department of Agriculture has started requiring companies that process and transport milk to test it for H5N1. But the CDC says that people should not be concerned about contracting bird flu from dairy products as long as they have been pasteurized, a standard part of the production process before milk goes on sale or is used in any other products in the U.S. Pasteurization involves heating milk to at least 154°F for a set period of time to kill pathogens, and 99 percent of the milk sold in the U.S. is pasteurized.

Raw milk can carry the virus.

There are people who are into raw (unpasteurized) milk and, depending on the laws of their state, get it at stores, buy it directly from farms, or acquire it through buying stock in a cow herd. Some feel it’s better for digestion or that it jibes with their unprocessed lifestyle. But health experts are warning that now is a particularly bad time to join the ranks of dairy truthers. 

One of the farms that has generated headlines in the current outbreak, Raw Farm of Fresno, California, specializes in raw milk. California Department of Food and Agriculture officials found bird flu pathogens in its products and quarantined the farm. Raw Farm voluntarily recalled two lots of products. 

One child who drank milk from the farm became sick with fever and vomiting, but physicians aren’t sure if the culprit was H5N1. Two pet cats that consumed products from Raw Farm tested positive for bird flu. 

Scientists are concerned about the virus mutating into something more dangerous to people. 

Right now, bird flu is rarely transmitted to humans. When it is, in most cases, the person catches it through contact with biological material fresh from an infected bird. Transmission from cows to humans is probably similar, though the large-scale jump of the virus from birds to bovines was unexpected and worrisome.

Like many viruses, H5N1 mutates and adapts as it replicates itself with new genetic material from its host organisms. A study published in Science by researchers at the nonprofit Scripps Research Institute recently suggested the variation now spreading among dairy cows is one mutation away from becoming adept at latching onto human cells. 

But the researchers told the Washington Post that bird flu is not well-suited to human cell receptors (the precise entryways for an infection), and there are reasons to think it would be a challenge for H5N1 to mutate to become a pandemic among people. 

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