HIV/AIDS: This is a "Four Fluids" type of a thing-- blood, semen, vaginal secretions, and breast milk. Condoms will be totally effective against this if you use them during intercourse, but it's kind of hard to use a condom when you're trying to inject something into yourself or breastfeed something.
Chlamydia: This is transmitted when semen or mucous membrane secretions come in contact with other mucous membranes, which is to say that condoms are very helpful. Also, you can get an infection in your eye from Chlamydia, so goggles and/or condoms are advised for oral sex involving a penis, and hand-washing is, as always, appropriate.
Gonorrhea: Essentially the same deal as chlamydia-- are we beginning to see a pattern here? That pattern being "wear a condom"? Good, because it's condom week.
Crabs: Remember in elementary school, when we had to learn about not spreading lice? This is kind of like that. Don't have sex if you have crabs, seriously. If you absolutely must, however, a condom will be somewhat helpful.
Syphilis: This one can be spread all sorts of ways-- anal, oral, and vaginal sex, to the fetus during pregnancy, even through kissing-- oh my! But a condom will also be very helpful in this situation.
The bottom line? Wear condoms and, of course, get yourself tested!