The sheer number of people with HES and the number of deaths is really unprecedented in any recent outbreak, De Waal said.
E. coli traditionally affects children and seniors, but this outbreak has disproportionately affected women, as well as people in their twenties, thirties, forties and fifties. Marler suggested two possible explanations.
"It may be that people consuming the product were all women in that age group, and they like to eat salads. It may be something about this particular bacteria, that it just simply may be a more virulent strain and attacking people you would not expect."
This article is reproduced with permission from PBS NewsHour. It was first published on June 6, 2011.