The bacteria H. pylori prefers an acidic environment but cannot survive the highly acidic stomach acid environment alone. It produces urease, an enzyme that converts urea in the stomach to ammonia. The ammonia helps neutralize the stomach acid around it so the bacteria can survive in the stomach.
Other parasites like tapeworms can only survive stomach acid in the egg form. The eggs are encased in fatty cysts in infected meat. The stomach acid dissolves the cysts and allows the tapeworms to hatch and infect the intestines when leaving the stomach.
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