2. Suppose that a certain disease strikes .01% of the population in a year, and suppose that occurrences of it are believed to be independent from person to person. Find the probability of three or more cases in a given year in a town of 20,000 people.

Note: This problem gives a slight flavor of the problems faced in the field of epidemiology.

They are often confronted with the difficult problem of determining whether an apparent cluster of persons with a specific disease is due to natural variation or to some unusual underlying common cause, and, if so, what that common cause is.