Ninety people go on a big trip. At one point, they had a chance to go swimming in a lake; sixty did, thirty did not. After the trip, 21 people got really sick: 15 of the swimmers, and 6 who did not swim. Estimate the risk of getting sick due to swimming in the lake. How many swims would it take to get sick by swimming, I. e. get sick and know "for sure" that it was the swimming that did it?