Attempting to write a disease into my story.

Okay, so, I just genuinely had a question for anyone who may have an answer. I'm trying to write into my story a contagious disease, fatal, and while I don't have to know what it is, I like personally having the explanation so I can explore and portray it better. At most right now, all I can picture is my family being mostly bedridden and out of work/school, coughing, sneezing, extremely weak. Kill some of them within at most 6 months, and at bare minimum maybe 4, even with treatment. Any ideas for the possible illness? Much appreciated!