The average around Europe was about one-third dead, with some place essentially depopulated. Prior to modern antibiotics, pesticides, and antiparasitics (mostly tapeworm), I wouldn’t be surprised at all if half the population had some kind of chronic infection or infestation. That doesn’t even include diseases cause by dietary deficiencies such as vit.C or iodine.
A lack of sanitary plumbing, the habit of digging wells next to cesspools, living in cramped quarters in crowded cities, and a lack of understanding the necessity of hygiene caused a large part of the population to be “under the weather” a good part of the time.
Alton Brown - “Good Eats” episode on water