My husband recently had Ehrlichiosis. I had never even heard of it before. It is caused by a tick-borne bacterium. I haven't seen him that sick in 30 years. His white cell count was so low they were afraid to put him in the hospital (where all the nasty germs are).
We have a good friend who went on vacation in Tennessee, and while she was there got a bug bite on her leg. She didn’t think anything of it and returned to Michigan. 18 months later, after doctors had been baffled, they finally figured out what it was. She had stage 4 Lyme disease and was sleeping 23 hours a day and taking heavy prescription drugs to deal with the pain and everything else. They put her on very strong IV antibiotics. It took over 10 years for her to get the meds right and to make a complete recovery. It’s very, very nasty stuff.
I'd bet there are still several more diseases to be diagnosed that find ticks as the vector. Lyme, Oklahoma Tick fever, RMSP, Ehrlichiosis, and who knows what....