Do you mean herpes? It is a genital disease. It is transferred when your partner already has it and you come in contact with an outbreak that they have. Prevent them? Don't have sex with someone that has herpes.
They look like blisters and you will be taking medicine to prevent outbreaks for the rest of your life. That is if you do catch herpes.
the injun is correct. You could also prevent catching herpes by using a condom.