PYEONGCHANG, South Korea — Lari Lehtonen, an Olympic cross-country skier from Finland, pulled his two sons out of kindergarten a month ago. They were not allowed to attend birthday parties. They were prohibited from crowded indoor spaces. They could have play dates, but only after a call to the friend’s parents.

This may sound like a peculiar style of helicopter parenting, but Lehtonen was not worried about his children — he was worried about himself. Specifically, he was worried about catching a cold, and he knew that the more time the lads spent around other children, the more likely they were to become little vectors of disease who could wreck his Olympics.

“Most of our friends, they know not to invite our boys if someone in their house is sick,” Lehtonen said. “But I call or I text to make sure.”

Cordoning off the children is just one of a few dozen stay-healthy strategies deployed by athletes here at the Olympics. A norovirus outbreak — 139 cases and counting as of Saturday — has dominated the headlines in the early days of these Winter Games. But the rhinovirus, the most ubiquitous cause of the common cold, is the challenger every athlete here has been dreading. Unlike actual rivals, this one is invisible, omnipresent and tireless.