This old couple go to the doctor every 6 months to make sure everything is OK. So, in they hobble and the wife is called in first. After a while, out she goes and in walks the husband. Once he's taken a seat and made himself comfortable, the doctor says " I have some bad news for you. Your wife may have either Alzheimers or AIDS" "Bloody hell!" says the bloke " That's pretty serious, but how can you not be sure?" " It's diagnostic" says the doc" However, I've given her a 20 minute start, if she arrives home before you do, don't shag her."