AIDS

AIDS is a horrible and deadly virus which you can get by doing it with a hooker in real life. But in 40k, however, it is spread by Slaanesh after it was made by Nurgle, who gave it to Slaanesh as a gift.

It is God's way of saying; "Fuck you." IRL. AIDS is a clever acronym meaning 'Anally Inflicted Death Sentence.'