Greebs has a great point. How many people get p*ssed off when the person they are sitting next to is coughing and spluttering all over you? Would anybody actually have the balls to tuen around and say "Hey, dude's sick. Get him/her out of here!". And chances are the lurgie being shared is going to hit you during your days off as well. If you're sick - stay the hell away!
However, It does become easy to tell who the "dodgy divers" are on the watch. You know the one who checks the day sheets to see how many staff are in for the next day (which usually happens to be Weekend). Or you get the "set-up" dive with this as well..........check the day sheet, see how many staff are on, then towards the end of the shift suddenly has symptom over-load.