In looking over the students' school records, you find that the 76 sick students have some things in common for the time when they were most likely exposed to the contamination (March 7-10):
All the students live on campus in one of the university's 4 dorms.
All drank municipal tap water from the dorms.
All ate in the school's food court, which is 1 of 3 cafeterias on campus.
Only a handful of the sick students have classes in the same buildings.