CARNAL NATION

You've got the basic concept down: condoms can protect you from some sexually transmitted diseases (STDs) but not others. Condoms offer good protection against diseases—like HIV, gonorrhea, and chlamydia—that actually travel in semen (including pre-cum) and women's vaginal or cervical fluids. As long as the semen stays in the condom, it can't get into anyone's orifices and cause infection. Likewise fluids from a partner can’t make contact with the vulnerable mucous membranes of the penis.

But other STDs aren't so easy to prevent. The reason makes sense when you think about it: condoms can only protect the areas they cover. Same goes for internal "female" condoms and dental dams. Some sex diseases, including syphilis and chancroid, cause open sores or ulcers, and contact with these ulcers can spread the germs. These sores might occur on the penis or inside the vagina or anus—and in those cases condoms can be protective—but they can also appear elsewhere in the genital or anal area, where condoms don't provide any coverage.

Herpes can spread through skin-to-skin contact even when there are no open sores. A recent study found that condoms reduce the risk of herpes transmission by 30%—hardly enough for comfort. Similarly, human papillomavirus (HPV) can be transmitted via contact with genital or anal warts that are too tiny to see without a microscope. Hepatitis B and C are probably spread via small amounts of blood in the genital or anal area, but sexual transmission of these viruses isn't completely understood.

The CDC and Planned Parenthood both provide good basic info about specific STDs and how to prevent them.

A head-to-toe rubber suit will indeed protect you from catching pretty much everything, but unless that's your particular kink, you don't have to go that far. Certainly, it's smart to use condoms to prevent HIV and lower the risk of other diseases. Talk with your partners about your respective sexual health, but don't forget that everyone doesn’t always tell the truth, and some people don't even know what they’ve got.

To make sure you don't fall into the latter category, it's a good idea for sexually active people who aren't in a stable monogamous relationship to get tested regularly for STDs—and immediately if you have symptoms. If you're too embarrassed to ask your doctor, most cities have public clinics that will do this cheaply. Fortunately, most STDs can be treated, and early treatment both offers the best chance for a complete cure and lowers the risk of passing the nasties along.