Of the ten outbreaks reported since 1983, nine have been in the U.S. The characteristics of each outbreak have been similar to that caused by an infectious agent.
Although a comparatively large outbreak (117 patients) occurred in 1996 in Fannin County, Texas, the largest outbreak (122 patients) was the original one in Brainerd, MN. There have been no secondary cases reported in any of the outbreaks, suggesting that the