Flu Fridays: So, what causes the flu?

Happy Flu Friday! Today we’re going to talk about what causes the flu.

You may have heard that flu is caused by a virus. You may even know that there are different kinds of flu viruses out there, like “H1N1” and “bird flu.” But do you know the difference between them? Let’s take a closer look at the influenza virus so that you’re prepared when flu is in the news.

A virus is a tiny (way too small to be seen with the human eye) infectious particle. Viruses need to get inside other living things to reproduce, so they don’t just infect humans. Viruses are found inside animals, plants and bacteria as well. Viruses come in different shapes, and each type of virus acts differently once it gets inside the host.

There are actually three types of viruses that cause the flu. They’re called influenza A, influenza B and influenza C. Scientists have found that influenza C usually causes a mild respiratory infection that is usually not very serious.

Influenza A and B are the typical causes of the “flu season” that happens every winter around the world. These two types of flu viruses cause more serious illness in humans, so let’s talk more about them.

Influenza virus image, courtesy
Centers for Disease Control and Prevention.

When seen under a microscope, influenza A and B viruses usually look similar — imagine a round ball covered in spikes. The “spikes” are proteins that stick out from the surface of the virus. There are two types of proteins on the influenza virus: H, which stands for hemagglutinin, and N, which stands for neuraminidase.

These protein spikes do two things in your body:

1. They help the virus stick to the cell it wants to attack.
2. They help your immune system recognize that something has invaded your body.

If you have been infected with the same type of flu before — or if you got your flu shot — the H and N proteins are like a “caller ID” for your immune system. Your body knows exactly what to do to help you fight off that type of the flu. If it’s a type of influenza virus with Hs and Ns that your body has never seen before, it’s harder for your immune system to fight it off. This is why some strains of the flu make us sicker than others.

Here’s why viruses like the ones that cause influenza are hard to fight: Viruses can change their protein “spikes” so that the human body doesn’t recognize what’s infecting it. Influenza B viruses change their spikes (also called mutation) somewhat slowly. But influenza A can mutate very fast, and there are many different types.

Because influenza A viruses change so quickly, we give them different names based on their proteins. In 2009, you might remember the “swine flu” outbreak was also called H1N1. (Yes, those are the same H and N proteins we talked about earlier!) This was a version of influenza A that hadn’t been seen in humans for many decades, and scientists were worried that it would cause a lot of people to become very sick, very quickly. Luckily, even with over 1 million cases in the United States, the H1N1 pandemic was not as deadly as originally feared. (Read more about the 2009 H1N1 pandemic from CDC.)

Quickly mutating influenza A viruses are also the reason that CDC recommends getting your flu shot on a yearly basis — because there are different strains of the flu virus every year. This season’s flu shot includes protection against influenza B as well as the H1N1 and H3N2 strains of influenza A.

If you haven’t gotten your flu shot yet for the 2011-2012 season, it’s not too late! You can read more about the vaccine, and find out where to get your shot, at Flu.gov.

Thanks for joining us for our first-ever Flu Friday! Remember, if you have any flu questions, email us at getready@apha.org.