1. What causes a sexually transmitted disease or sexually transmitted infection (STD/STI)?
There are two major causes of STDs/STIs:
Bacteria, including chlamydia, gonorrhea, and syphilis
Viruses, including HIV/AIDS, herpes simplex virus, human papillomavirus (HPV), hepatitis B
virus, and cytomegalovirus (CMV; pronounced sahy-toh-MEG-uh-loh-vahy-ruhs), yeasts and
protozoan parasites, such as Trichomonas vaginalis (pronounced TRIK-uh-MOH-nuss vaj-uh-
NAHY-lis), or insects such as crab lice or scabies mites, cause STDs/STIs.1
Any STD/STI can be spread through sexual intercourse, and some STDs/STIs also are spread
through oral sex and sex play. Ejaculation does not have to occur for an STD/STI to be passed
from person to person. Sharing contaminated needles used to inject drugs or using contaminated
body piercing and tattooing equipment also can transmit some infections, such as HIV or hepatitis
B and C.
A few diseases (such as CMV and molluscum contagiosum) can be sexually transmitted but are
also spread through nonsexual, close contact. Regardless of how a person is exposed, once a
person is infected by an STD/STI, he or she can spread the infection to other people through oral,
vaginal, or anal sex, even if he or she has no symptoms.
STDs/STIs are of particular concern in pregnant women, because some infections can be passed
on to the infant before birth or during delivery. However, the risk of transmission from mother to
infant can be lowered, and it is important for every expectant mother to be screened.
For example, HIV can be passed from mother to infant during pregnancy before birth, at the time
of delivery, or after birth during breastfeeding.2 This transmission can be prevented through
treatment with certain medications during pregnancy and near delivery. After birth, women who
have HIV should refrain from breastfeeding their infants if safe alternatives, such as infant
formula, are available, further reducing the infant's risk.3
In other cases, if the mother has an infection such as gonorrhea or herpes, in which risks of
transmission are high during delivery, other steps can be taken to reduce the likelihood that the
infant will be infected. In these instances, health care providers can treat the pregnant woman for
the STD/STI before birth, or the infant can be delivered by cesarean section (also referred to as C
section).3
CMV affects about 1% of all births. A pregnant woman infected with CMV can transmit the
infection to the fetus in the womb, or it can be passed to the infant during delivery or by
breastfeeding. She could also pass it to her newborn after birth if the child comes into contact
with her body fluids (for example, saliva or urine) carrying the virus. If a health care provider
suspects that a woman has a CMV infection during pregnancy, an ultrasound examination, blood
tests, and other tests are done to assess the health of the fetus. Most infants who were infected
with the virus during pregnancy do not have any detectable problems after birth. But, 10% or 20%
will have serious problems, including deafness and intellectual disabilities. If fetal testing shows
increased risk of serious problems, some women opt to end the pregnancy. Researchers are
studying antiviral drugs, immune treatments, and other medical approaches to control the
infection during pregnancy. Some research is focusing on vaccines to prevent CMV infection.4