What Is the Flu (and Why Is It So Difficult to Contain)?

Spoiler: There are different types of flu that can make you sick.

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Despite all the prevention efforts, the flu ravages through communities and causes millions of Americans to miss work and school—and even lands over 200,000 people in the hospital—each year. What makes influenza so unruly?

The flu is caused by the influenza virus. It enters through your mouth or nose, causing a respiratory infection. Symptoms of the flu include fever, body aches, and fatigue, and sometimes cough, runny nose, and other cold-like symptoms.

Symptoms typically improve after two to five days, but they can last a week or more. While these symptoms might not be a big deal to most people, the flu can lead to serious complications like pneumonia, especially for people who are already dealing with a chronic medical condition.

The Ever-Changing Types of Flu

Even with the annual flu vaccine, the virus is hard to contain each year. Part of that is because of insufficient vaccination rates, leaving millions of Americans vulnerable to catching and spreading influenza. But the other reason the flu is difficult to contain is that the flu comes in numerous types and strains.

Types of influenza are A, B, C, and D, but A and B are the most common and dangerous for humans. (Influenza D typically only affects cattle, and influenza C only causes mild respiratory symptoms).

Influenza A can be further divided into dozens of subtypes based on two proteins on the surface of the cell: hemagglutinin (H) and neuraminidase (N). Influenza A viruses are named based on these two proteins (for example, the currently circulating “H3N2” virus, or the infamous “H1N1” virus that caused a severe outbreak in 2009). There are 18 subtypes of hemagglutinin, and 11 subtypes of neuraminidase, meaning there are 198 possible subtypes of influenza A alone.

Influenza B doesn’t have these subtypes, but it still comes in many different strains and lineages (based on where the virus first originated).

What’s important to know about the influenza virus—regardless of the type—is that the numerous strains are constantly shifting and going through genetic changes.

In other words, there’s not just one single way to get infected with the flu. This is why the flu vaccine needs to be reformulated every year, in order to match the shifting strains that are expected to circulate that year. Some strains are easier to match than others, causing variation in the vaccine’s effectiveness from year to year.

Preparing an effective flu vaccine is tricky business, and while it’s not yet “perfect” and isn’t a bulletproof defense against the flu, it’s still one of your best weapons against the virus. The flu vaccine reduces your risk of the flu by 40 to 60 percent (depending on the year). Plus, those who get the flu despite being vaccinated tend to have less severe symptoms and shorter flu duration. (Luckily, researchers are working on a universal flu vaccine that could have longer and more broad protection against the virus.)

Now that you’ve learned the basics about the flu, find out if you’re up to speed on these 10 flu facts.