We've all heard of "the flu" but when it comes to types, subtypes and variants, influenza can get a bit complicated.
According to the U.S. Centers for Disease Control and Prevention (CDC), nearly 9,000 people were admitted to hospitals with influenza in the week ending December 14 and 9 percent of clinical tests were positive for influenza.
Both of these statistics are up from the week before, indicating that the U.S. is heading further into flu season.
What Is Influenza A?
Influenza is a common respiratory virus that is categorized into four types: A, B, C and D. Type C only causes mild infections and type D is not believed to infect humans.
However, influenza A and B can infect humans and contribute to seasonal flu epidemics: winter months during which there is a high rate of infection.
Influenza B primarily affects humans but mutates slowly, so populations are likely to have some immunity against infection.
In contrast, influenza A affects many different animals, including birds and humans, and mutates quickly. This means new strains can emerge that infect people and spread efficiently, without as much protection from our immune systems. For this reason, influenza A is the only type that causes pandemics.
Influenza A is believed to be the most severe type of flu virus for humans, although this is the subject of debate, as there is some evidence to suggest that influenza B might be equally as dangerous.
How Does It Differ From Seasonal Flu?
When flu season hits, it is both the influenza A and B variants that are responsible for the resulting illnesses.
In the most recent influenza surveillance report from the CDC, of the week ending December 14, 96.7 percent of flu infections were caused by influenza A.
Influenza A is broken down even further into many different subtypes defined by the proteins on each virus's surface.
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 are present.
For instance, the dominant circulating strains of influenza A among humans are H1N1 and H3N2. This means that one contains hemagglutinin 1 and neuraminidase 1 and the other contains hemagglutinin 3 and neuraminidase 2.
Is It Related to Bird Flu?
Bird flu 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 per the name, but has adapted to infect cows and can infect other species in spillover infections.
A spillover infection happens when an animal has contact with an animal of a different species that is infected with a virus such as bird flu, and becomes ill themselves, but cannot pass the virus onto another of their own species.
For example, a human working on a farm with infected chickens may become ill with bird flu but would not pass the virus on to another person.
The 2024 outbreak of bird flu has infected 65 people in the U.S. so far, according to the CDC, but there have been no recorded cases of human-to-human transmission.
What Are the Symptoms?
The symptoms of influenza A vary from person to person and depend on subtype, but generally consist of coughing, runny or stuffy nose, sneezing, sore throat, fever, headache, fatigue and body aches.
People infected with bird flu may also experience conjunctivitis, an eye infection sometimes known as "pink eye."
In severe cases, the flu can develop to diarrhea, nausea, vomiting, dizziness, difficulty breathing, pneumonia and even death.
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