probability measures how dangerous something is. It uses a math equation to find the risk that a certain event might happen. In epidemiology, risk is defined as the number of events that "did" happen, divided by the number of events that "could have" happened. For example, if the risk of babies dying after birth in a population is reported as 1/1000, it means that out of every 1000 babies born, 1 is likely to die.