In most cases STDs (Sexually Transmitted Diseases) and STIs (Sexually Transmitted Infections) are transmitted through sexual activity involving the transfer of sexual fluids or skin-to-skin contact.
How are STIs Spread?
STIs can be spread several different ways. Infections can be sexually spread through vaginal, anal or oral intercourse, or contact without penetration (e.g. chlamydia, syphilis). Other STIs, like herpes or genital warts, can be spread through skin-to-skin contact only (not needing penetration or ejaculation). Still others, such as HIV or hepatitis B or C, can be spread more than one way: through sexual contact, or through blood from an infected person getting into the blood of an uninfected person. This can happen when needles or rigs are shared to use drugs.
HIV, hepatitis B and C, herpes and syphilis can be passed from an infected mother to her fetus or newborn baby. STIs can be passed from woman to man and vice versa, as well as from man to man or woman to woman. Public health providers see many people who got an STI – particularly, gonorrhea and syphilis – through unprotected oral sex. Often, people are unaware that they can get STIs through oral sex.
Source: STD Resource.com.
For more information, visit the Centers for Disease Control and Prevention website.