If petting the lizard is the source of the contamination, how do you explain student number 7?

A preschool class visited the zoo. As part of the trip, they had a chance to pet a large lizard and then had lunch. Some students brought lunch from home; others bought lunch at the zoo. That evening four students became ill.

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Student 1 2 3 4 5 6 7 8 Pet the Lizard Yes No No Yes Yes No Yes Yes Lunch Home Home Home Zoo Home Zoo Home Zoo Sick Yes No No Yes Yes No No Yes