What Is the Flu, Exactly?

This informational guide, part of POPSUGAR's Condition Center, lays out the realities of this health concern: what it is, what it can look like, and strategies that medical experts say are proven to help. You should always consult your doctor regarding matters pertaining to your health and before starting any course of medical treatment.

Influenza is a contagious viral infection that affects your nose, throat, and lungs. Most people can recover from a case without lasting problems, but it can make you feel pretty lousy while you're fighting it off. It's about twice as likely to infect people under age 65 as those who are older. But older people, along with pregnant people, are more likely to develop severe symptoms from the flu; it can even be fatal.

Between three and 11 percent of people come down with the flu every year, and while influenza viruses are in the environment year-round, they are most common in the fall and winter, which is why flu activity usually peaks between December and February (in the Northern Hemisphere). "The severity of the virus dictates how bad a flu season it is," says Paul Adamson, MD, an infectious-disease doctor at UCLA's Geffen School of Medicine. In other words, the strains that circulate may be more contagious, or may cause more intense symptoms, in some years than others. And while the virus can mutate from year to year, the basic symptoms remain pretty similar: fever, chills, cough, sore throat, body aches, headache, runny nose, and fatigue are all signs of the flu. Another telltale sign is the rapid onset of symptoms — one second you feel fine, and the next you're sick.

People with the flu are most contagious for the first few days they're ill, so it's important to stay at home when you suspect you have the flu — but since symptoms may not appear for a day or two after you've been exposed, you may be able to infect others before you even know you're ill, according to the Centers for Disease Control. That's why vaccination is so important. The best way to prevent the flu is by getting the annual flu vaccine, Dr. Adamson says. In general, vaccines reduce the risk of flu among the population by 40 to 60 percent. Each year's shot is developed based on the previous year's strain of the virus, so if a strain mutates significantly one year, the new vaccine may be less effective. But even when that happens, being vaccinated can still help to prevent severe illness, experts say.

The virus spreads mainly through the air; infected people expel tiny respiratory droplets that contain the virus when they cough, sneeze, or talk. It's also possible to catch the flu from touching a surface that has the virus on it, then touching your nose or mouth — but this is less likely than airborne transmission.

While anyone can get very sick with the flu, certain people are at higher risk of developing dangerous symptoms.

It's especially important for these groups to get vaccinated and, when possible, avoid crowded places during peak flu season in order to protect their health.

Most people can get through a bout of the flu with rest and plenty of fluids, such as water, juice, and soup, to prevent dehydration, according to the Cleveland Clinic. If you're achy, you can take an over-the-counter pain reliever, like acetaminophen. But if your symptoms begin getting worse instead of better or you're at high risk for flu-related complications, see your doctor for other treatment options. "People who are at risk of severe illness are good candidates for antivirals, but you need to take them within the first couple of days of symptoms," Dr. Adamson says. "It can help reduce the severity of the illness." Pregnant people can safely take Tamiflu, the most-often-used antiviral, as well.