Most of the answers here don’t address the question. The number 1 reason is that you ingest the cyst form of the parasite, which is dormant and highly resistant to all environments. Once it gets to your intestines it will grow into a trophozoite which is the full grown organism.
In addition, man bacteria create toxins on the food *before* you ingest it. The bacteria will die in your stomach but the toxins will still exert their effect.
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