Let's say there is this disease that affects someone with a probability of .001. This foolish scientist tests people, and under their test someone tests positive with a probability of .001, regardless of if they have the disease or not. Would the probability of a false positive (positive when not infected) be .001, and the probability of a false negative (negative when infected) be .999, so would the test have .001 + .99 = 100% error? This doesn't quite make sense, since there have to be a lot of people who are not infected and do not test positive under this random test.