Two of the three girls in the Longview story have leukemia, not aplastic anemia. The rate of new leukemia cases in the US is about 1 per 10,000 per year, so Longview will on average see 5 new cases each year. (
http://en.wikipedia.org/wiki/Leukemia#Epidemiology)
What are the chances that somewhere in the US, there will be a school where two girls of the same age get leukemia in the same year? Pretty good, actually. It would be a different matter if the same thing had happened last year, and every year for the past five years, or if it happens again next year, and every year for the next five years. So far, that is not the case in Longview.
The case of aplastic anemia is different, because that is a rarer diagnosis, and there have been seven cases in Cowlitz and Columbia counties over the last 10 years. With a population between the two counties of about 150,000, this is equivalent to about 5 cases per million per year. The standard rate is given as something between 2 and 10 cases per million per year, so if it is unusual, it is not dramatically so.
What would be truly unusual, given the persistent rates of a number of diseases in the US, is if no one in Longview ever got sick. We can only wish it were so.
Steve
