How to Know If You Are Contagious

This is an elevator sneeze. The snot that someone wiped on this handrail an hour ago. Your colleague, who will not stay at home , breathes next to you. Cold and flu viruses are everywhere. And if something falls for you, you also distribute them.

“There is evidence that the [flu] virus starts before symptoms appear,” says Dr. Pat Salber , an emergency room physician in San Francisco. For example, studies on ferrets (which can become infected and transmit the human flu) show that they can infect their ferret friends in a nearby cage even before they start showing symptoms such as a cute little ferret sneezing.

You are most infectious about three days after you first catch the flu, which is usually the first or second day of your symptoms. If you are smart (and your company’s sick leave policy is reasonable), you will probably already be in bed at home. But you can also transmit the virus, even if you have a super-strong immune system that fights it off successfully. One UK Flu Watch cohort study found that 77 percent of people infected with the flu had no symptoms . So you never know – the person who infected you with the flu may be someone you never knew existed.

For most of us, the flu is contagious for about a week. By the time you feel better, you will probably stop spewing viral particles all over the place, according to Dr. Salber. You can still be a little contagious, but doctors usually don’t consider you a big risk. “Usually – as it is now, in the context of a lot of influenza – you probably aren’t the only one spreading it.” (Wonderful.)

The flu virus is better understood than hundreds of different viruses that cause the common cold, so the details can vary depending on what exactly you are sick with and how your specific immune system is handling the situation.

And it’s not just sneezing: virologist Ian McKay writes that numerous studies show that some of us can catch colds and flu just by breathing . These studies mainly focus on influenza and RSV (one of the most serious common cold viruses), but it is likely that we breathe all kinds of viruses. “[This study] is broadly applicable to all 200+ respiratory viruses,” he says.

Oh yeah, that snot-covered handrail? McKay notes that we touch surfaces and then mouth or nose about three to four times an hour. He writes: “It is a good habit to refrain from touching our faces until we can wash our hands.”


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