Still the worst thing you can contract is rabies. Only virus with a 99.999999% mortality rate in humans once symptoms start. Depending upon where the virus enters your system you have 1 to 30 days to get treatment. If you don't, you are pretty much dead. I remember reading that the reason it's not 100% is that there is only 6 people to ever survive rabies after the onset of symptoms and that is it. And those survivalists were with the Wisconsin protocol method which has an 8% effectiveness.
My "favorite" horrific disease is fatal familial insomnia. It's an incredibly rare prion disease that destroys the part of your brain which allows you to sleep. People who have it suffer from increasingly worse insomnia until they go crazy and eventually die.
