There have been a few instances, IIRC there was an Apollo mission that had a head cold spread among the whole crew.

But that's unlikely to be the case here because they've been up there isolated for over 6 months now

This describes NASA's pre-flight quarantines since Apollo 14, and makes (very brief) references to in-flight diseases (and apparent transmission) prior to Apollo 14 ("upper respiratory infection"; "viral gastroenteritis"),

https://www.nasa.gov/wp-content/uploads/2023/12/ochmo-tb-006... ("Health Stabilization Program (HSP)")