Final answer:
STIs are diseases spread through sexual activity, caused by pathogens like bacteria and viruses; prevention is best achieved through abstinence or a committed monogamous relationship.
Step-by-step explanation:
Sexually transmitted infections (STIs) are diseases spread through sexual activity. The best way to prevent them is abstinence or a mutually monogamous relationship where both partners are not infected.
STIs are caused by pathogens, which are organisms such as bacteria or viruses, that can infect the body.
These pathogens typically spread through direct contact between mucous membranes and are also transmittable through body fluids like blood, semen, and breast milk. While using protection such as condoms during sexual activity can reduce the risk of STIs, they are not completely foolproof as infections can still be transmitted through areas not covered by condoms or due to incorrect usage of condoms.