Flu season is hitting earlier and harder this year than last—and myths about how it spreads could make you sick. For example, many people think colds and flu are only contagious after the symptoms hit.

Others go to great lengths to avoid contact with anyone who has recently been ill. Why risk sitting next to a coworker who has just returned to the office after a nasty bout of the flu, even if he’s no longer sneezing and coughing?

Here’s a look at when flu is most contagious—and the best ways to protect yourself and your family.

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26 Times More Flu Cases than Last Year

Since September, the CDC has reported more than 22,000 cases of flu, compared to just 849 cases in the same period a year ago. The surge of infections has prompted Boston to declare a public health emergency, and a Pennsylvania hospital was so swamped that it had to put up a tent to handle flu patients.

Officials are predicting a potentially severe flu season nationwide, with widespread infections already reported in 41 states, according to the CDC. Drug companies are reporting shortages of kids’ formulations of Tamiflu, the leading treatment and delays in shipping most forms of flu vaccine—making it particularly crucial to take extra precautions to avoid catching or spreading the disease.

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When Does Flu First Become Contagious?

Like other viral illnesses, flu has an incubation period before its characteristic symptoms—including fever, chills, runny nose, headaches, achy muscles, fatigue, and sore throat—strike. During the interval between exposure to flu and onset of illness—which typically lasts one to four days—levels of the virus rapidly multiply inside the body.

According to the CDC, most adults with flu can infect others starting one day before symptoms strike. In addition, some people who are infected with flu don’t develop any symptoms, but can still transmit the virus during the period while it’s active in their body. That means you can transmit the infection before you know you’re sick or catch flu from someone who looks and feels perfectly healthy.

Each year, 5 to 20 percent of Americans catch flu, which is responsible for more than 200,000 hospitalizations and up to 49,000 deaths annually.

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When Is Someone With Flu Most Likely to Infect Others?

Most adults with flu are contagious from one day before symptoms start until five to seven days after the symptoms appear. However, some people—including kids as well as adults with weakened immune systems—may remain contagious for up 14 days after they start to feel sick.

In general, the risk of spreading flu is greatest when the person’s symptoms are at their worst, because levels of the virus are likely to be at their highest point. Later in the disease, the immune system’s counterattack decreases the risk of infection.

However, the infected person should still be considered contagious for one week after the illness starts (for most adults) or for up to two weeks in the case of kids and people with a weakened immune system.

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How Does Flu Spread?

Flu is mainly an airborne infection. A flu carrier (with or without symptoms) can transmit the disease from a distance of up to six feet through airborne droplets released when the person coughs, sneezes, or talks. Other people catch flu if the droplets land on their mouth or nose or if they inhale the infected spray.

You can also catch the disease by touching something that an infected person has contaminated either by handling it (such as an ATM screen or elevator button) or by coughing or sneezing near the object.

What Are the Best Ways to Fight Flu?

The CDC recommends these three steps:

•Get a flu shot. The CDC recommends vaccination for everyone age 6 months or older, unless there’s a medical reason (such as allergy) not to get the shot. As I reported recently, a surprising benefit of flu shots is that they reduce heart attack risk.

•Take precautions to stop the spread of germs. To protect yourself and others, wash your hands frequently with soap and water and cover your mouth when you sneeze or cough. If you come down with a flu-like illness, stay home until at least 24 hours after the fever is gone.

•Take antiviral drugs, if advised by your healthcare provider. These prescription drugs can make flu milder and reduce the number of days you’re sick, reports the CDC. Studies show that they work best if taken within two days of falling ill, but may still help if taken later in the illness.