How do you prevent a Summer cold?

This joke plays on the common advice about preventing colds, which are more associated with the winter season due to the cold weather. The humor comes from the literal interpretation of “catching” a cold as if it were an object that could be caught at a specific time. The joke suggests that to prevent a summer cold, one should simply “catch it” in the winter instead, as if timing the sickness is a matter of choice. It’s funny because it subverts the usual concern about avoiding colds altogether by implying you can schedule when to get sick, which is, of course, not how colds work.