If our stomach contains a highly corrosive acid, how do viruses and parasites survive to be able to infect us?
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Although not all viruses and parasites *do* survive to be able to infect us, when they do, it can be by way of the same chemical processes that protect our stomach. The stomach uses a mucus lining to protect itself; likewise, in the case of [influenza viruses](https://academic.oup.com/jid/article/216/1/105/3814252), our nasal mucus actually incidentally protects the viruses as they pass through the gastrointestinal system.
But there are other possibilities too. In the case of [the tapeworm](https://www.eurekalert.org/news-releases/652811), they have proteins that block the activity of digestive enzymes to prevent themselves from being digested. This also accounts for some of the negative side-effects of a tapeworm infection such as malnutrition.
Interestingly, some parasites [seem to have developed](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6860651/) to actually require exposure to corrosive stomach acids before they can hatch. Examples at that link includes the protozoan diarrheal parasite *Giardia* (which requires stomach acid exposure for excystment), as well as for the hatching of the eggs of the nematode *Ascaris lumbricoides*.
This requirement for acid exposure is similar to the [scarification](https://en.wikipedia.org/wiki/Scarification_(botany)) required for the germination of many plant seeds; it helps the parasites time their emergence from the egg, so that the next stage of their development occurs in the proper environment.
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