It's good to read this page first if you're new to sex. There are very important concepts on this page. BUT, the individual sections on each STD are hyperlinked to this page. So don't worry about memorizing all of these words before reading the page you're interested in. These are like your body's ammunition against invading bugs.
Opportunistic infections are also an example of a complication.
For example, when a cut gets infected, it turns red, hurts a lot, and white pus comes out. For example, in oral sex, the mouth is in intimate contact with the genitals. In dry humping, two people's genitals directly touch each other. In vaginal intercourse, the penis directly touches the vagina. Contact between two vaginas is also intimate skin contact.