According to the World Health Organization, more than 30 types of bacteria, viruses and parasites can move from person to person as a result of sexual activity. Among the most common are:
Fluid exchange is behind the transmission of most of these diseases during sex. Condoms physically block the fluids, preventing disease spread. Although condoms can break or have manufacturing defects, latex doesn't have pores that can allow microorganisms to pass through [source: FHI].
However, some STIs, including genital warts, genital herpes and pubic lice, are present in and on an infected person's skin or hair, areas that aren't covered by condoms. These conditions can be transmitted even with perfect condom use.
So what does that boil down to in terms of actual risk? UNAIDS reports a study of serodiscordant couples -- one person in the couple is HIV positive, and the other is HIV negative. Over a two-year period, the people in the study who used condoms correctly for every sexual encounter had almost no chance of contracting HIV. The participants who were inconsistent in their condom use had a 14 to 21 percent chance of contracting HIV.
The statistics for pregnancy prevention are similar. Over the course of a year, a woman using a condom correctly for every act of sexual intercourse with a man has a 3 percent chance of becoming pregnant. With typical use -- which isn't perfect -- the risk of unplanned pregnancy is 12 percent [source: WHO]. A woman using no contraception has an 85 percent chance of becoming pregnant [source: Trussell].
Condoms' health benefits are clear -- but some argue that making them available encourages promiscuity. One study suggests that this is not the case. The study observed 4,018 teenagers between 1994 and 2002. By the end of the study, all of the participants were sexually active. The teenagers who used condoms during their first sexual intercourse were not more likely to have had more partners than those who didn't. However, the condom-using teens were less likely to have been diagnosed with chlamydia or gonorrhea [source: Shafii et al].
Condoms' ability to prevent the spread of disease makes them an important tool for public health -- which has a big impact on how they're marketed.