Joint probability is the probability of two events occurring in conjunction -that is, the probability that event A and event B both occur, written as ) or P(AB) and pronounced A intersect B. The probability of someone dying from the pandemic flu is equal to the joint probability of someone contracting the flu (event A) and the flu killing them (event B). Joint probabilities are regularly used in Probabilistic Risk Assessments and Event Trees.
Sample Usage: The probability of developing a fever from influenza is equal to the joint probability of someone contracting influenza and developing a fever.
Source: DHS Risk Lexicon, U.S. Department of Homeland Security, 2010 Edition. September 2010 Regulatory Guidance