You're right, and I was wrong; parasites are less common than bacteria as sources of stomach upset. I'm afraid I fell into the trap of confusing my personal experience with what is generally true. After your post I did a bit of my own research, which confirmed the essence of what you said.

My direct experience has been that presumed bacterial GI stuff usually self-resolves, while giardiasis lasts and lasts without resolution, responding only to anti-parasitic meds like metronidazole. I've also had amoebic dysentery, which has a somewhat different presentation but worse potential outcomes, and is also effectively treated with metronidazole (and similar, like tinidazole). Note that these experiences involved South Asia--specifically Nepal and India--and rural West Africa. I've had no similar experiences elsewhere in Africa, Asia, Europe, or the Americas.

On extended travels I carry antibiotics from a couple of different families (usually a fluoroquinolone and a penicillin), an anti-parasitic like metronidazole, plus a raft of other meds and first aid supplies. I'm a big fan of self-diagnosis and treatment, but of course official advice is to consult medical professionals at every opportunity--none of these meds are without side effects, some serious. Metronidazole, for example, is considered a carcinogen.

As for that research, here are reliable estimates for annual waterborne infections in just the USA: Otitis external ("swimmer's ear"), 4,670,000; Norovirus, 1,330,000; Giardiasis, 415,000; Cryptosporidiosis, 322,000; Campylobacteriosis, 171,000. (The same source stated an estimated 219 annual *deaths* from swimmer's ear, suggesting that none of these is to be taken lightly.) In any case, although norovirus is three times as common, there are nearly a half million cases of giardiasis--and this is in the USA, where water supplies are generally quite safe.

My apologies for posting without first checking my assumptions, and my thanks for disagreeing calmly and respectfully! Wishing you continued safe journeys and boundless enjoyment!

Mark