Unprotected receptive anal sex (with an HIV positive partner) is the sex act most likely to result in HIV transmission. Other infections that can be transmitted by unprotected anal sex are human papillomavirus (HPV) (which can increase risk of anal cancer); typhoid fever; amoebiasis; chlamydia; cryptosporidiosis; E. coli infections; giardiasis; gonorrhea; hepatitis A; hepatitis B; hepatitis C; herpes simplex; Kaposi's sarcoma-associated herpesvirus (HHV-8); lymphogranuloma venereum; Mycoplasma hominis; Mycoplasma genitalium; pubic lice; salmonellosis; shigella; syphilis; tuberculosis; and Ureaplasma urealyticum.
Some infections can cause problems during pregnancy, such as Giardia (giardiasis) and Group B Streptococcus (GBS), among others. Giardia can cause chronic diarrhea which may lead to dehydration and malnutrition, both of which can be dangerous during pregnancy both for the mother and fetus. GBS may be transmitted to the baby during delivery and can lead to an infection in the infant. Both Giardia and GBS often colonize the rectum and can be spread to the vaginal tract if vaginal or oral sex follows anal sex, or if there is any touching (hands or genitals) after anal sex.
^ Joann S. DeLora; Carol A. B. Warren; Carol Rinkleib Ellison (2008) . Understanding Sexual Interaction. Houghton Mifflin (Original from the University of Virginia). p. 123. ISBN 978-0-395-29724-7. Retrieved November 6, 2011. Many men find anal intercourse more exciting than penile-vaginal intercourse because the anal opening is usually smaller and tighter than the vagina. Probably the forbidden aspect of anal intercourse also makes it more exciting for some people.
Intra-rectal pressure builds as the rectum fills with feces, pushing the feces against the walls of the anal canal. Contractions of abdominal and pelvic floor muscles can create intra-abdominal pressure which further increases intra-rectal pressure. The internal anal sphincter (an involuntary muscle) responds to the pressure by relaxing, thus allowing the feces to enter the canal. The rectum shortens as feces are pushed into the anal canal and peristaltic waves push the feces out of the rectum. Relaxation of the internal and external anal sphincters allows the feces to exit from the anus, finally, as the levator ani muscles pull the anus up over the exiting feces.
Spreading STIs. Infections and diseases that are shared during sexual intercourse — such as HIV, gonorrhea, chlamydia, and herpes — can be shared through anal sex. In fact, anal sex is the sexual behavior for transmitting and getting HIV for both men and women. People on the receiving end (or “the bottom”) of anal sex are more likely to become infected with HIV than the inserting partner (or “the top”).