When you feel a sneeze or a cough coming on, covering your mouth prevents the spread of infectious germs. You probably knew that.
But the way you cover up also matters, and there are plenty of people who have not yet heard the consensus guidance of health officials: If no tissue is available, you should aim into your elbow, not your hand. Even if that means breaking a long-held habit.
"If somebody sneezes into their hands, that creates an opportunity for those germs to be passed on to other people, or contaminate other objects that people touch," said Dr Vincent Hill, chief of the waterborne disease prevention branch of the Centres for Disease Control and Prevention (CDC) in the United States.
Germs are most commonly spread by the respiratory droplets emitted from sneezing and coughing. When they land on your hands, they are transmitted to things like doorknobs, elevator buttons and other surfaces people around you are likely to also touch.
Sneezing and coughing into your arm has become the standard suggestion of not just the CDC, but also organisations like the American Academy of Paediatrics and the American Public Health Association.
Even the New York City subway system occasionally runs an announcement asking riders to "cough or sneeze into the bend of your arm or use a tissue".
The suggestion is relatively new. The CDC guidance has become official only in the last 10 to 15 years, Dr Hill said.
Dr Georges Benjamin, executive director of the American Public Health Association, said he began seeing the suggestion more prominently about 10 years ago.
Children are frequently taught in school the proper way to cough or sneeze - sometimes referred to as the Dracula cough, since it makes you look like the count covering up with his cape.
University of Missouri-Kansas City School of Medicine professor of paediatrics Mary Anne Jackson said the term "cough etiquette" first turned up in 2000, and she traced the suggestion to sneeze into your arm to 2003, when Sars fears were widespread.
It gained further prominence in 2009, when the H1N1 swine flu pandemic struck the US.
That year, Ms Kathleen Sebelius, then the Health and Human Services Secretary, shamed NBC journalist Chuck Todd for his sneezing etiquette at a White House press briefing. (Rush Limbaugh, the conservative radio host, later dismissed her advice as coming from "elitist snobs".)
To be clear, the manoeuvre does not eliminate all risk, even if it is the best tactic available.
Studies have shown that even masks cannot prevent all droplets from becoming airborne, Prof Jackson said.
But anything to reduce the amount of flying particles helps. And health officials keep coming back to a drum that can never be beaten enough: Make sure you consistently wash your hands.
"Hand washing is one of the most important things people can do to keep healthy," Dr Hill said.