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

Flu symptoms can last up to two weeks. But that doesn't mean you have to spend it all in isolation.

• People who have the flu can be contagious the day before they start showing signs of the virus.
• You're generally most infectious during the first two or 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.

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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 people who are feeling feverish, coughing, and sneezing — and others generally feeling miserable — are heading to hospitals to get a flu diagnosis and prescription drugs that can help the illness end faster.

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

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

James Steckelberg, a Mayo Clinic doctor who's an expert on infectious diseases, told Business Insider that it was tough to dole out concrete advice for how long to stay home, as 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 returning to your 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 found that most "viral shedding" — when you're really passing the germs around — with influenza occurs in the first two or three days after a person gets sick. Kids can often be contagious for more than seven days.

People typically have the flu for five to seven days, but some can stay sick for up to two weeks. Researchers say Day 2 tends to be the worst for symptoms, but that can vary.

Anyone within a six-foot radius can give you the flu, according to the CDC, especially if that person is coughing, sneezing, or talking. That kind of person-to-person transmission is more likely to cause an infection than touching a surface with some flu virus on it and then reaching for your nose or mouth.

But it's still important to wash your hands thoroughly and avoid touching your face. Wearing a face mask can also help prevent you from getting the flu if someone you live with catches it.

Experts have said the flu vaccine is not as effective this year as it has been in the past at protecting people from a common strain, but that's it's still a good idea to get the shot because it can protect you from others. Flu season can last into May, so if you haven't gotten your dose yet, it's not too late.

Flu symptoms can last up to two weeks. But that doesn't mean you have to spend it all in isolation. Read Full Story