HIV can be transmitted in different ways. Blood transfusion is by far the riskiest: if you receive HIV+ blood, the chance you aquire HIV is 90%. The chance that HIV is passed from a HIV-positive mother to a baby during childbirth is 25%.
Although the risk per act is low, most people are infected by HIV through sexual contact or exposure to infected body fluids or tissues, because of frequency of these behaviors. We can calculate your risk of acquring HIV if you are have these risky behaviors. You need to answer 7 questions.