Represents a variety of illnesses, organisms, and syndromes.
Sex, I suppose, although being a widower (male) increases the risk for an STD in the first year after the wifes death, esp if they have viagra. Go figure.
The more aggressive the patient is at pubic hair grooming and shaving, the greater the risk of STI (PubMed).
Depends on the infection of which you are concerned. Follow the CDC guidelines.