Universal Precautions generally refers to the set of guidelines set forward by the CDC (Center for Disease Control) to prevent the transmission of disease via body fluids. In the case of the body modification industry, this primarily means blood, but can also include semen and other fluids. It does not generally refer to feces, nasal secretions, sweat, tears, or urine unless blood is visible in these. While it does not apply to saliva in and of itself, in the case of body modification, any saliva encountered will quite likely be contaminated by blood.
To quote from CDC literature:
"Under universal precautions, blood and certain body fluids of all patients are considered potentially infectious for human immunodeficiency virus (HIV), hepatitis B virus (HBV), and other bloodborne pathogens."
All body piercing and tattoo studios are expected to adhere to universal precautions, and most new legislation enforces this expectation.