It’s all that summertime swimming in those deep, fresh water lakes. A lot of runoff nowadays from agriculture (pesticides, manure sludge) also non-native algae and other organisms have been introduced over the past couple of decades that have altered the ecosystems. This all contributes to tummy trouble.

Sorry, not the cause.

Even if you eat it? Give me another order of deep fried mole crickets and a palm rat sandwich with a warm glass of reclaimed water to wash it down.

I think the post specified swimming, not drinking. But I suppose drinking untreated contaminated lake water, while not recommended, could gain one a nasty case of cryptosporidiosis or worse.

I don't have your qualifications, but I remember from my youth swimming in the ocean or a public pool and getting a mouthful of water that was swallowed reflexively and ending up with a tummy ache. There is some nasty stuff out there that can be inhaled, swallowed or absorbed without knowing.

there can be.

I just read that gorgeous crystal blue Lake Tahoe in California has been invaded by a algae because of the unusually warm temperatures. A woman’s dog died after swimming in the lake and the veterinarian said he thought it was due to the algae. In Vermont, Lake Champlain has had a terrible problem with algae and bacteria and now the cities along the lake issue warnings about swimming in it. My friend developed nasal fungal balls her doctor attributed to her daily swims in the lake during the summer.