The disease I have in mind is 'consumption' or as it is known today, tuberculosis. I realize this can be easily cured nowadays, but in the story, they couldn't cure it and have to the conclusion that is was a new, tougher strain of the disease.
Edit: The story is set mostly in the present time (modern day) and a bit on the early 1900s. *smacks self* I'm sorry for the confusion. Thank you so much for your help!
EDIT (again): THank you so much to those who helped! Now I have a better idea of what to write!