Final answer:
The STI that causes sores and sometimes fever on the genitals, mouth, or rectum is genital herpes, which is caused by the herpes simplex virus.
Step-by-step explanation:
The sexually transmitted infection (STI) that infects the genitals, mouth, or rectum, causing outbreaks of sores sometimes accompanied by fever is genital herpes. Genital herpes is caused by the herpes simplex virus and can cause painful blisters in the affected areas. These blisters can heal on their own; however, the virus remains in the body, and outbreaks can recur, especially under stress or illness. Transmission can occur even in the absence of visible sores, making it important for sexually active individuals to practice safe sex and undergo regular STI screenings.