In the story I'm currently writing, one of the characters falls ill and eventually dies from their sickness. In the time that they are sick, the character becomes so weak that they can't move or talk. What are some diseases that cause paralysis, extreme pain, or anything that could prevent a person from moving while sick? This story takes place in the 1930s-1940s, so a disease like this that was common back then is preferred.
Polio comes to mind immediately. It would have the effects you are looking for and is fairly well known and easy to research. The time period you are planing on would also fit it perfectly, as those are around the years of the American polio endemic.
Just yesterday I saw a documentary in which there was a young woman who had paralyzed legs from measles. Without modern medicine she would probably have died.
There are also all sorts of weird parasites that could cause paralysis and other illnesses. Paralysis: What Is It, Diagnosis, Management & Prevention (clevelandclinic.org) Causes of paralysis - Reeve Foundation (christopherreeve.org) What Causes Paralysis? (news-medical.net)
