In a small lab hidden in a basement, a young genius had just finished crafting his greatest revenge on the human race yet. All his life he had been rejected by every girl he ever had any interest in, but last week he had just been turned down rather harshly and it was the last straw. He decided then and there that if he couldn't find someone, he would reduce the whole world into horny, sex driven slaves of their own desires.
Today was the day, he had finished his virus, once infected the virus would not only change the body of the host, but transmit to the next person through sex and change them in one of two ways. The first way being the outcome 75% of the time would be to slowly change them to be more attractive to the person who infected them, the second way being just a carrier where it would act as if it was a new host being freshly infected, all freshly infected individuals would slowly become more attractive based on what they thought would make them attractive.
All infected will have a few side effects/symptoms, one being they will become more and more horny until they have sex, and if they go long enough without their bodies would start to change to try to attract a mate, these changes may or may not be permanent depending on how long and how strong the effect was.
Another odd side effect was the sexual juices from an infected was so saturated with the virus that it was a 80% chance of infection, but a 100% chance of causing some sort of slow change to areas touched. A common example of these being that a man having sex with an infected woman would find his penis to be growing at roughly 1 inch per 10 minuets of contact. hardly noticeable at first, and likely he wont notice until he is infected. The rate of change increases greatly for those already infected as it basically just speeds along the changes.
All that was left was to unleash this on the world, but where would be a good start...