Hello there!
This fact is an example of history effect.
Step-by-step explanation:
History effects refer to events or circumstances generated in the surroundings that influence or change an experiment, affecting its result within the internal validity method of an investigation. These effects can occur at the beginning, in the middle, or at the end of the experiment.
This case shares an experiment of the eating habits of American people towards spinach. After two weeks, the experiment causes an outbreak of the E. Coli bacteria found in the spinach causing the experiment's main source to be the reason for its end as well.