Students will come to the classroom after PE class. 15 people drank pure water from the drinking fountain and 5 of them soon developed diarrhea. The pure water in the drinking fountain was immediately sent for testing and the test results were not sure that there were harmful substances that caused diarrhea. 
The argument of the subject is flawed, because it regards the lack of evidence to prove the existence of a situation as sufficient evidence to prove that it does not exist.
 drinking pure water from the drinking fountain is not the cause of diarrhea.