Sexually transmitted infections are caused by one of 3 types of organisms.

Parasites: Parasites are very small animals that get nourishment from that the person they infect. The infected person is called the host. Pubic lice, also called crabs, are an example of sexually transmitted parasites.

Bacteria: Bacteria are organism made of one cell. They cling together to feed from the body of their host. Gonorrhea, syphilis, chlamydia, and chancroid are examples of sexually transmitted bacteria.

Viruses: Viruses are complex molecules that can multiply when they invade that cells of their host. HIV, herpes, and hepatitis are examples of sexually transmitted viruses.

Usually infections that are caused by a virus cannot be cured, but in some causes they can be controlled.

Parasitic infections can be treated with medications or over–the–counter drugs. Bacterial infections can be treated with antibiotics
STIs are usually caused by bacteria or viruses. Your chances of getting them are highest if:

• You have unprotected sex
• A Condom tears during intercourse
• Your sexual partner has an STI
• Your partner is engaging in sex with other people

In some cases, such as with HIV or hepatitis B, viruses can also be spread through infected blood or shared needles and syringes, including those used for drugs, body piercing, or tattoos. Pregnant women can pass certain infections (e.g., HIV) on to their babies prior to or during birth, or while breast-feeding. Growth of genital warts caused by HPV may appear during pregnancy because of changes in hormone levels.