Wikipedia
Serosorting, also known as "serodiscrimination", is the practice of using HIV status as a decision-making point in choosing sexual behavior. Frequently the term is used to describe the behavior of a person who chooses a sexual partner assumed to be of the same HIV serostatus in order to engage in unprotected sex with them for a reduced risk of acquiring or transmitting HIV/AIDS.
Knowledge of HIV status is based on the result of a person's HIV test, with a positive result indicating that a person has HIV, and can potentially transmit the disease to others during any sexual contact involving an exchange of bodily fluids (e.g., unprotected anal or vaginal sex). There are many situations where determining their partner's serostatus outside of clinical settings cannot be done with complete certainty, limiting the efficacy in mitigating the transmission of HIV/AIDS (or other STDs). As people do not typically engage in sex practices with the expectation of contracting or transmitting HIV, failed attempts at serosorting are a leading cause of the contraction of HIV among partners.