Why is abstinence the only certain way to prevent STDs?

If you are having sex, you're in SOME risk of contracting a sexually-transmitted infection, or STI. Avoiding sex - abstinence - completely is the only almost-certain way of avoiding them.

Even that isn't COMPLETELY certain. What we call "sexually transmitted" means that the infection is very difficult to catch; only the most intimate contact will transmit the disease. Sex is pretty intimate - but blood transfusions are just as "intimate" in this sense. If the blood donor has an STI, it's possible to catch it that way.

Like sex, but still want to minimize the risk of an STI? Have an exclusive, monogamous relationship with your partner, who has never had sexual contact with anyone else, and never stray.