I have a few live recordings where quiet, slow songs or delicate instrumental pieces are marred by the sound of someone coughing in the audience, and I've been at a couple of concerts where I really wished that whoever was coughing, sneezing and blowing their nose had stayed home.

Aside from the risk of infecting others (I don't know when the exact contagious period is, I'll leave that to the scientists) if it's a singaround or folk club evening it's likely distracting to the rest of the audience and the performer to have someone coughing, sneezing, and blowing their nose   

There are some situations where it's unavoidable (if someone can't miss work because they don't have sick leave, for example), but attending a singaround or folk club would appear to be an entirely optional activity.

Also, I can't imagine that somebody in the throes of a bad cold would enjoy themselves very much anyway - better to stay home with a hot toddy.