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The stomach bacterium Helicobacter pylori is well known as a cause of ulcers and stomach cancer. it also protects against oesophageal cancer – the same strains account for both these pros and cons. H. pylori is neither a good nor a bad microbe; it’s both
https://aeon.co/essays/there-is-no-such-thing-as-a-good-or-a-bad-microbe