This year's deadly flu season is reaching its peak - here's how to tell if you're contagious

Shutterstock

• Flu sufferers can be contagious the day before they start showing signs of the virus.
• You're generally most infectious during the first two to three days of the illness.
• Experts say you can head back to work or school when you've been fever-free for a full 24 hours.

Flu experts agree that this season is shaping up to be nasty in the US. In California alone, 27 people under the age of 65 have died from the flu, and 26 states (plus New York City) are now reporting high flu activity.

More fever-sufferers, coughers, sneezers, and generally miserable people are heading to hospitals to be diagnosed with the flu and get prescription drugs that can help the illness end faster.

But not everyone needs to see a doctor. Many flu-ridden people can stay home and wait out the symptoms with the help of liquids, over-the-counter drugs, and a lot of sleep. But that begs the question: How do you know when it's safe to head back into the office or return to school?

Scientists who study the virus say influenza can incubate in the body for one to four days before you show any signs of getting sick. That means you can be contagious the day before you start feeling terrible, and have no idea that you're spreading the virus to your colleagues, as the Centers for Disease Control and Prevention points out.

James Steckelberg, a doctor and infectious disease expert at the Mayo Clinic, told Business Insider that it can be tough to dole out concrete advice for how long to stay home, since everybody reacts a little differently to the flu.

But his advice jibes with the CDC's: Wait until you've been fever-free (without the help of drugs) for 24 hours before heading back into the office or school. That means no more chills, sweats, or flushed appearance.

Other research suggests the first few days of the sickness are the most dangerous. A 2008 study in Hong Kong showed that most "viral shedding" (that's when you're really passing the germs around) with influenza occurs in the first two to three days after you get sick. Kids can often be contagious for more than seven days.

People typically suffer from the flu for for about a week (5-7 days), but some can stay sick for up to two weeks. Researchers say day two tends to be the worst when it comes to symptoms, but that can vary.

Anyone within a six foot radius can give you their flu, especially if that person is coughing, sneezing, or talking, according to the CDC. That kind of person-to-person transmission is more likely to cause an infection than simply touching a surface with some flu virus on it then reaching for your nose or your mouth. But it's still important to wash your hands thoroughly and avoid touching your face with your hands. Handwashing and facemasks can also be effective barriers to illness if someone you live with catches the flu.

The flu vaccine is not as effective this year as it has been in the past, but it's still a good idea to get vaccinated because the shot is very effective at preventing certain strains. Flu season can last into May, so if you haven't gotten your dose yet, it's not too late.