Based on a Department of Health advert in New Zealand, where a mentally handicapped child sits on a swing, rubbing his eye, and suffering from Rubella. Thereby, any evidence of one element (stupidity, rubbing eye, having rubella) would be met with the other two. For instance, if someone gets a basic question wrong (what is one times one), you rub your eye and shout rubella at them.
Conversely, if someone rubs their eye with their forefinger, it is a sign that they are stupid, and have rubella. If someone actually had rubella, presumably they'd be rubbing their eye and being retarded like in the advert, so we never bothered with that one.