What Is the Flu?

What the flu is may surprise you. It’s not one single virus. There are several types and frequent mutations. That’s why you need to get a flu shot every year.

The odds are you, or someone you know, has had the flu at some time or another. Also called influenza, the flu is a common, contagious respiratory infection.

It primarily causes symptoms in your nose, throat, and lungs:

• Cough
• Congestion
• Runny nose
• Chills
• Fever
• Muscle aches
• Fatigue
• Headaches

A case of the flu can be fairly mild or downright miserable. For seniors, young children, pregnant women, and people with weak immune systems or chronic diseases, the flu can be serious and even deadly, the Centers for Disease Control and Prevention (CDC) points out.

It’s important to understand that there are different types of flu. What’s more, the flu, which typically strikes in late fall and winter, frequently mutates. As a result, flu vaccines are “tweaked” annually to offer protection from the latest strains of flu expected to strike the populace during what is commonly called flu season. That’s why it’s important to get a flu shot every year.

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Understanding flu viruses

There are four types of influenza (flu) viruses: A, B, C, and D. Types A and B cause seasonal outbreaks of the infections best known as “the flu.” Type C typically results in mild forms of influenza. Type D influenza affects animals, primarily cattle, but it is not known to infect humans.

Influenza A viruses are the most problematic type of the flu. They are the only varieties of influenza known to cause global flu epidemics. Epidemics occur when a new subtype of influenza A virus develops through mutations and not only infects people but also is typically highly contagious, spreading quickly from country to country, the CDC explains.

(It is important to note that “the flu” is not the COVID-19 virus).

A case in point: In the spring of 2009, a new influenza A (H1N1) variant was detected in the U.S. Researchers discovered the virus, officially called influenza A (H1N1)pdm09, contained influenza genes never before seen in flu strains previously known to infect people or animals.

Laboratory testing, however, revealed the virus contained gene segments similar to influenza viruses that had been most recently identified in, and known to circulate among, pigs — so that flu strain was referred to as swine flu. It quickly spread throughout the U.S. to other countries, primarily sickening children and young people.

Although the World Health Organization (WHO) declared the global H1N1 swine flu pandemic over in August of 2010, the (H1N1)pdm09 virus continues to circulate as a seasonal flu virus. The good news is flu vaccines are regularly updated to match flu viruses that might be circulating, including the (H1N1)pdm09 variant.

Can you catch the flu from animals?

So far, researchers have identified more than 100 type A influenza virus subtypes in nature. The main subtypes that circulate in the human population include influenza A H1N1 and H3N2. Other type A flu viruses are found in many different animals, including ducks, chickens, pigs, whales, horses, and seals.

While the flu viruses found in animals don’t normally infect humans, there have been some notable, if relatively rare, exceptions. Scientists call these “variant viruses” sporadic human infection with a flu virus that normally infects only animals. Most human infections with variant virus have occurred in people who worked closely with sick pigs infected with various forms of swine flu viruses.

Zoonotic influenza refers to flu caused by animal influenza viruses that cross from animal to humans, the WHO explains. In addition to the swine flu virus subtypes that can infect people, types of “bird flu” — avian influenza virus subtypes A H5N1 and A H9N2 — have been reported.

Bird flu is very rare and, in most cases, has not passed from person to person, the National Institutes of Health (NIH) points out. In case that should change, scientists are working on new vaccines that will offer protection.

It’s important to note that, while other animal species, including dogs and horses, sometimes contract their own varieties of flu viruses, those forms of influenza are distinctly different from the human flu and not something pet owners should fear catching.

The best way to protect yourself from the flu is to wash your hands frequently, avoid people who are sick and — most importantly — get a flu shot. The optimum time get the flu vaccine is by the end of October because it takes at least two weeks for the shot to start working, according to the NIH.

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