Condoms

Condoms only provide protection when used properly as a barrier, and only to and from the area that it covers. Uncovered areas are still susceptible to many STDs. In the case of HIV, sexual transmission routes almost always involve the penis, as HIV cannot spread through unbroken skin, thus properly shielding the insertive penis with a properly worn condom from the vagina and anus effectively stops HIV transmission. An infected fluid to broken skin borne direct transmission of HIV would not be considered "sexually transmitted", but can still theoretically occur during sexual contact, this can be avoided simply by not engaging in sexual contact when having open bleeding wounds. Other STDs, even viral infections, can be prevented with the use of latex condoms as a barrier. Some microorganisms and viruses are small enough to pass through the pores in natural skin condoms, but are still too large to pass through latex condoms.

Condoms are designed, tested, and manufactured to never fail if used properly. There has not been one documented case of an HIV transmission due to an improperly manufactured condom

Proper usage entails:

Not putting the condom on too tight at the end, and leaving 1.5 cm (3/4 inch) room at the tip for ejaculation. Putting the condom on snug can and often does lead to failure.
Wearing a condom too loose can defeat the barrier.
Avoiding inverting, spilling a condom once worn, whether it has ejaculate in it or not, even for a second.
Avoiding condoms made of substances other than latex or polyurethane, as they don't protect against HIV.
Avoiding the use of oil based lubricants (or anything with oil in it) with latex condoms, as oil can eat holes into them.
Using flavored condoms for oral sex only, as the sugar in the flavoring can lead to yeast infections if used to penetrate.
Not following the first five guidelines above perpetuates the common misconception that condoms aren't tested or designed properly.

In order to best protect oneself and the partner from STIs, the old condom and its contents should be assumed to be still infectious. Therefore the old condom must be properly disposed of. A new condom should be used for each act of intercourse, as multiple usage increases the chance of breakage, defeating the primary purpose as a barrier.