Herpes
Herpes or herpes simplex virus is an STI of the lips (on the face) and the genitals. There are two kinds of herpes viruses, called oral herpes and genital herpes. Oral herpes is also called HSV-1 and genital herpes is also called HSV-2.
Oral herpes mainly affects the face (and sometimes the throat). When active, oral herpes causes blisters on the lips that are contagious. When dormant there are no symptoms.
Genital herpes causes blisters on the genitals that can break open and form small ulcers. Genital herpes is infectious when active.
I have heard that there is still a chance for herpes to spread even if it is dormant. I have also heard that oral herpes can infect genitals and the other way around, though that's rare.
The only way to prevent infection is not to have vaginal sex, oral sex, manual sex, or anal sex.
There is no cure for either oral herpes or genital herpes and there is no vaccine to prevent infection.