Parasites Plagued Roman Soldiers at Hadrian's Wall

- Transmission is easier in the city due to closer contact and more shared resources.

- Urban parasites are likely to be more debilitating to the victim because they may come from an unfamiliar environment. (Compare how hookworm in the American south was a nuisance to blacks, but debilitating to whites.)

We know that diseases were a much heavier burden in cities than they were in rural regions. Parasites are mostly just bigger diseases; you'd need to come up with a really interesting idea to explain why they were a smaller issue in cities than outside of them.

Hookworm is an interesting example to consider here; you catch them by stepping on soil with your bare feet. Stereotypically they are a problem of the rural south. But I found this paper on "neglected tropical diseases" in the United States, which had this to say:

> Toxocariasis is a soil-transmitted helminth infection [it isn't hookworm, but hookworm is also a soil-transmitted helminth infection] that can result in visceral larva migrans, visual impairment from ocular larval migrans, or a condition that resembles asthma, known as covert toxocariasis. Urban playgrounds in the US have recently been shown to be a particularly rich source of Toxocara eggs, and inner-city children are at high risk of acquiring the infection.

https://journals.plos.org/plosntds/article?id=10.1371/journa...

Another example to consider might be covid, where my read of the consensus is that (1) initial nature -> human infection is more likely in rural areas (where there's more nature), but (2) once it can live in humans, it's a bigger problem in urban areas (where there are more humans).

So, I doubt that immune system theory, since for most of mankind’s existence, they were not part of our life.