Dr. Besser knew E.coli could move from animals to humans. Perhaps the children had touched animals that crried the bacteria? Besser found other E.coli cases in the area where the children lived. But it wasn't enough.
Basser then made a list of what the sick children had eaten. They had all eaten cheese, apple juice, and fish. He then made a list of what healthy children in the area had eaten, and compared his lists. They had eaten the cheese and fish, but not the apple juice.