STI in full is Sexually Transmitted Infection. STD in full is Sexually Transmitted Disease. The 2 words refer to the same thing-illnesses passed by sex. The small difference is that an infection means that there are germs in the body which do not always cause symptoms, whereas a disease has physical symptoms. To avoid STIs/STDs abstain or always use condoms when you have sex or be faithful to one partner you have tested with.
Attribution: Straight Talk Foundation