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Infectious Disease A study considered risk factors for HIV infection among intravenous drug users [11]. It found that 40% of users who had s 100 injections per month (light users) and 55% of us- ers who had > 100 injections per month (heavy users) were HIV positive. 4.54 What is the probability that exactly 3 of 5 light users are HIV positive? 4.55 What is the probability that at least 3 of 5 light users are HIV positive? 4.56 Suppose we have a group of 10 light users and 10 heavy users. What is the probability that exactly 3 of the 20 users are HIV positive