We’ve all heard of “the flu,” but when it comes to types, subtypes, and variants, the flu can be a bit complicated.

According to the US Centers for Disease Control and Prevention (CDC), nearly 9,000 people were admitted to hospitals with the flu in the week ending December 14, and 9 percent of clinical tests were positive for the flu.

Both of these statistics are up from the previous week, indicating that the US is heading further into the flu season.

What is influenza A?

Influenza is a common respiratory virus that comes in four types: A, B, C, and D. Type C causes only mild infections, and type D is not believed to infect humans.

However, influenza A and B can infect people and contribute to seasonal flu epidemics: the winter months during which infection rates are high.

Influenza B primarily affects humans, but it mutates slowly, so populations probably have some immunity to infection.

In contrast, influenza A affects many different animals, including birds and humans, and mutates rapidly. This means that new strains can emerge that infect humans and spread effectively without our immune system providing as much protection. For this reason, influenza A is the only type that causes pandemics.

Influenza A is considered the most serious type of flu virus for humans, although this is up for debate as there is some evidence to suggest that influenza B may be just as dangerous.

How is it different from the seasonal flu?

When flu season hits, it’s both influenza A and B that are responsible for the resulting illnesses.

In the CDC’s latest flu surveillance report for the week ending Dec. 14, 96.7 percent of flu infections were caused by influenza A.

Influenza A breaks down even further into many different subtypes defined by proteins on the surface of each virus.

Specifically, there are 18 possible types of a protein called hemagglutinin (H) and 11 types of neuraminidase (N); influenza subtypes are labeled depending on which of these proteins is present.

For example, the dominant circulating strains of influenza A among humans are H1N1 and H3N2. That is, one contains hemagglutinin 1 and neuraminidase 1, and the other contains hemagglutinin 3 and neuraminidase 2.

Is it related to bird flu?

Avian influenza is another version of influenza A. Its full scientific name is avian influenza A(H5N1), so it contains hemagglutinin 5 and neuraminidase 1.

Bird flu primarily affects birds, as the name suggests, but it has adapted to infect cows and can infect other species by spillover.

A spillover infection occurs when an animal comes into contact with an animal of another species that is infected with a virus, such as bird flu, and becomes ill itself, but cannot transmit the virus to another of its own species.

For example, a person working on a farm with infected chickens can contract bird flu, but will not pass the virus on to another person.

According to the CDC, the 2024 bird flu outbreak has infected 65 people in the U.S. so far, but no cases of human-to-human transmission have been reported.

What are the symptoms?

Influenza A symptoms vary from person to person and depend on the subtype, but generally consist of a cough, runny or stuffy nose, sneezing, sore throat, fever, headache, fatigue, and body aches.

People infected with bird flu can also experience conjunctivitis, an eye infection sometimes known as “pink eye.”

In severe cases, the flu can develop into diarrhea, nausea, vomiting, dizziness, difficulty breathing, pneumonia and even death.