A few times reading UK-based historical texts about the 18th and 19th centuries (previously in reading about the Grand Tour and Romantic era continental tourism, recently in a throwaway line in an Eric Hobsbawm book), I've run across a mention that, away from the coasts, Italy was depopulated, almost deserted. The context definitely seems to be that there were once people living in these places, and there were ruins of former "civilization", but that these areas had become deserted. It's almost always mentioned that these areas were "malarial", and implied that the depopulation happened because of endemic malaria.
Is this true? And if it's true, is the above context (depopulated rather than naturally underpopulated, depopulated due to some kind of "unhealthful conditions" vs. other reasons) accurate? Are there other historical reasons that this part of Italy was relatively deserted in this era?
Note: I've traveled extensively in Italy and know that interior Italy is largely mountainous, and even today these areas are fairly underdeveloped and remote. So having a smaller population makes sense to me. That it would be noticeable by British visitors 200+ years ago and framed as this land being incompatible with life and health is the part that surprises me.
Without reading the original texts you’re referencing and citing the specific things they recounted, I’d say there are two phenomena your British writers could be referring to.
The first is a long secular decline: After the post-Roman transformation, there was a broad trend in Italy whereby population migrated away from the south to the north. The south had formerly been the busier part of the peninsula for much of classical history: it was closest to the “ancient” epicenter of mediterranean trade in the east (the triangle formed by Greece, the Levant, and Egypt) so much so that large parts of the Italian south were mostly greek-speaking, and given the Empire’s Italian cities could be easily supplied with grain from Egypt and North Africa, it was also more suitable for growing cash crops like olives and grapes, making it a desirable place to invest for wealthy urbanites. As a final plus, it was closer to the empire’s capital (Rome), facilitating investment and attention from the ruling class.
In the post-roman period, the fertile flatlands of the north (more suitable for intensive agriculture) began attracting population as Southern Italy’s ties to the Eastern Mediterranean waned. Northern cities were also closer to the new monarchies which emerged over the Alps, making them a more attractive place to live as Europe's balance of power shifted northward. Eventually, Southern Italy’s political and economic ties also turned from the Eastern Mediterranean to the West, stimulating the growth of western-facing harbors (like Naples and Palermo) at the expense of eastern-facing ones, further stimulating population decline in formerly populated areas.
So there definitely was a phenomenon whereby in the south, formerly bustling communities which in the past had been propped up by a couple of nearby Latifundia and commercial links to the east over the centuries had devolved into small hamlets (with only ancient ruins to testify their former prosperity). As communities shrank, this created a self-reinforcing cycle: irrigated fields were abandoned and water stagnated, so malaria emerged as problem which further drove depopulation. On the other hand, in the north the inverse happened: small roman-era hamlets had grown into bustling communities as agriculture intensified nearby. Intensive irrigation works in the Lombardy and the Veneto kept Malaria mostly at bay, even in cities with significant waterways like Milan and Venice.
Secondly, we also need to keep in mind that the British writers were visiting Italy in the 18th and 19th centuries, just as the industrialization process was beginning. We won’t forget that Italy was a late industrializer and an inefficient one at that (especially in the south) but the process by which people accelerated moving into urban spaces was active all the same. At the time, Naples and Palermo were the two largest and fastest-growing cities in Italy, attracting even more people from the countryside than they had in past centuries. This lopsided economic development, especially in the south, would have been shocking to well-to-do travelers from Northern Europe, where increasingly economic prosperity was less concentrated (for example: the British Midlands and the Northwest were important centers of the industrialization, but Southern Italy developed no such “Inland” prosperity).