Sexually transmitted infections. Studies that examined risk and protective factors related to STIs were divided into those that looked at symptoms (n ¼ 7 studies) and those in which an objective diagnosis of a particular STI was reported (n ¼ 11). Only two key risk factors and no key protective factor emerged for symptoms of STIs, and only individual-level key risk and protective factors were identified for an STI diagnosis. Anal sex, prostitution, and previous history of an STI were all strongly associated risk factors. The key protective factor that emerged was condom use, which was reported to be protective in two studies [47,48]; one study found that never-use of condoms increased the risk of herpes simplex virus-2 by four times [49] (Table 6).