How to Know If You’re Contagious

That sneeze in the elevator. The snot somebody wiped on that handrail an hour ago. Your coworker who won’t stay home, breathing right next to you. Cold and flu viruses are everywhere. And if you’re coming down with something, you’re spreading them too.

“There is evidence that you start shedding the [flu] virus even before you get symptoms,” says Dr. Pat Salber, a San Francisco-based emergency room internist. For example, studies with ferrets (who can catch and transmit human flu) show that they can infect their ferret friends in the next cage over, even before they start showing symptoms like cute little ferret sneezes.

You’re most contagious about three days after you first get a flu virus, which tends to be the first or second day of your symptoms. If you’re smart (and your company’s sick policy is a sensible one), you’ll probably be home in bed at that point. But you can also transmit the virus even if you have a super strong immune system that’s successfully fighting it off. One study from the UK Flu Watch cohort found that 77 percent of people infected with the flu had no symptoms. So you never know—the person who gave you the flu might be someone you never suspected.

In most of us, flu is contagious for about a week. By the time you’re feeling better, you have probably stopped spewing virus particles everywhere, Dr. Salber says. You might still be a little contagious, but doctors don’t usually consider you a big risk. “Usually—like right now, in the context of lots of flu—you’re probably not the only one spreading it around.” (Wonderful.)

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The influenza virus is better studied than the hundreds of different viruses that cause colds, so the details may vary depending on exactly what you’re sick with, and on how your particular immune system is handling the situation.

It’s not just sneezes, either: Virologist Ian Mackay writes that plenty of research suggests some of us can transmit colds and flu just by breathing. Those studies mostly concern flu and RSV (one of the more serious cold viruses), but it seems likely that we’re breathing all kinds of viruses. “[This research is] broadly applicable to all 200+ respiratory viruses,” he says.

Oh, and that snot-covered handrail? Mackay points out that we touch surfaces, and then our mouth or nose, about three to four times per hour. He writes: “Holding off on touching our faces until after we can wash our hands is a good habit to develop.”

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