if you want a non-copy pasted answer: Bacteria in the intestinal tract helps us digest food; the production of cheeses, yogurts, sausage, etc. depend on bacteria as well as fermenting bacteria in order to help them achieve their finished states; bacterial infections in the human body often relies on 'harmless' bacteria to fight against the harmful bacteria; they are used in the manufacturing of antibiotics (such as Pseudomonas, antibacterial, etc.). However, they can also be detrimental, including bacteria found in untreated water, which can result in sever harm to the digestive tract, local plant-life, and can cause flu-like symptoms upon ingestion.