That wasn't just a rumor.

First, if we take "public toilets" to mean the location rather than the plumbing, that is a location where people inject heroin (sharing needles) and sometimes fuck.

Second, if we take "public toilets" to mean the plumbing itself, the seats may be contaminated with fresh bodily fluids of all types. (mainly blood for women or semen for men) You could rub against it with your cunt, open sore, dickhole, prolapsed anus, or eyeball. Some seats even have splinters/slivers to jab you in the ass, or in the ballsack or tongue or whatever.