Final answer:
STIs are caused by microorganisms that can live in and on reproductive organ surfaces and in various bodily fluids, including blood.
Step-by-step explanation:
Microorganisms that cause STIs live in and on the surfaces of the reproductive organs and in a person's blood or other bodily fluids. These infectious agents include bacteria and viruses that enter the body through the mucous membranes of the reproductive tract during sexual activities such as vaginal, an_al, and oral sex. Additionally, certain STIs can spread through the exchange of body fluids like blood, semen, and breast milk, making activities like sharing needles for drug injection or tattooing potential routes of transmission. Hence, the correct answer to the question is 'Choice #1 and #3'.