This week, we're continuing our look at the 'Fremen Mirage', the notion that 'hard times' breed 'strong men' and 'good times' create 'weak men' - this time by tracing the intellectual history of the idea, starting in the ancient Mediterranean.https://acoup.blog/2020/02/07/collections-the-fremen-mirage-part-iiia-by-the-princess-irulan/ …
Not saying Europe wasn't a backwater in the Middle Ages - in many ways it was. But I see no reason to assume that European 'immune systems' were appreciably stronger than, say, Chinese ones. The 'germs' part of Guns, Germs and Steel only works in the New World.