Does ALL occur in clusters?

There've been suggestions that ALL (Acute Lymphoblastic Leukemia) occurs in space and time clusters, which should show to infectious origin of the disease at least in a certain part of causes.

I've data of ALL diagnoses in the Czech Republic in years 1997 - 2004 (and there should be more if we found significant results here) and I'm trying to find out whether there are some traces of clustering or not.

But I've to find out something about data clustering, cluster analysis itself and so on first.