STDs, short for Sexually Transmitted Deceases, are one of the very unfortunate realities. STDs get their name because they are usually transmitted during sexual contact, however with BDSM there are many ways STDs can be transmitted without actual intercourse. For more information what types of STDs exist and how you can have relatively save intercourse, see Wikipedia.
Many STDs, including HIV and hepatitis, can be transmitted through blood. This means that you should always wear gloves and be very careful during any activity that includes blood. Needles, crops or anything else that came in contact with blood needs to be cleaned and disinfected after use. Never use these kinds of toys on more than one person at the time.
Body fluids are another way STDs can be transmitted. Urine and semen can carry some STDs. Like with blood, avoid direct contact with these fluids using gloves and other barriers.
Leather is one of the most common materials in bondage. The textured surface of leather easily harbors STDs. Especially fungi will survive on leather (or even grow there) easily. Unfortunately, because leather is a natural product, all disinfectants will deteriorate the material. If you plan on using leather toys with many different people, you will need to rub it down with alcohol and accept that the material will become ugly and break quickly.