Diseases over time mutate. People might not understand the exact mechanism but they do change. And the symptoms of a disease may or may not be the same from one period to the next.
Take Rheumatic fever. I am currently researching scarlet fever as a character has contracted it before the start of the book. I was going to use polio but it wasn't really fully understood until approximately the 1830/1840s and as this story takes place in 1814, I want to have something where the boy has a chance of living.
Scarlet fever is one of those things you read about -- think Little Women or the Velveteen Rabbit, but once I started to do my research I discovered that it is not scarlet fever that is the problem, but rather the rheumatic fever that follows. And around 1800, the nature of that disease changed -- rather than simply just attacking the joints, it began to attack the heart and the St Vitus dance or chorea appeared. Suddenly over half of the victims developed heart problems. Most recovering within six months. Then it took an even worse turn in the early 20th century. Anyway I would hazard a guess that part of the changing nature of the disease down to the over -crowded conditions. Several hospitals owe their existence to wealthy benefactors founding them AFTER their children died. It is a disease that is mainly confined to children aged 5 -15.
Today, thanks to modern medicine, the incidences of rheumatic fever have been severely cut. Antibiotics form a good line of defence as does simple measures such as covering your mouth and nose when you sneeze and washing your hands before eating.
I found it intriguing though how it changed through out history. But having discovered the answers I need, I now need to write the book, instead of led down ever increasing labyrinths of research.