• Several types of bacteria can enter your body through contaminated food or water and cause diarrhea. Common bacteria that cause diarrhea include Campylobacter , Escherichia coli (E. coli), Salmonella , and Shigella. Hence, Escherichia coli, Shigella sp. and Salmonella sp. are all responsible for diarrhoeal diseases. 
• Dehydration takes place when your body loses more fluid than you drink. The most common cause of water loss from the body is excessive sweating and loss of fluids. Hence,  Dehydration is common to all types of diarrhoeal diseases and adequate supply of fluids and electrolytes should be ensured.

So, the correct answer is 'Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion'.