For the last few weeks, we've had more staff out sick at my library than usual. Infirmaries vary, but this week, whatever is going around really hit us - on Tuesday alone, almost half of the staff was out.
I called in sick Monday and Tuesday, and have been getting by with a lingering cough since then.
Personally, I think a bad enough cough is a good enough reason to call in sick if you work at a public service desk. Even if you feel fine, coughing makes helping someone an unpleasant experience for everyone. (Maybe I'm biased because this means more days at home reading for me, but I don't think I'm off base here).
My library has an informal policy of, "if you're sick, stay home so you don't infect everyone else." Of course we scramble to cover all the desks at times like this, but it's probably better in the long run to have a few people out for a few days than a lot of people out for weeks on end.
I'm sure studies have been done on this very topic, but I'm not feeling well enough right now for that kind of research.