One of the worst possible illnesses one can have is fatal familial insomnia. This disease has been rated as the worst because it cannot be treated and is passed from parent to child through gene mutation. The disease also has severe symptoms.
I don't think there's a worse. There are so many horrible illnesses that we don't even know about. I was watching Monsters Inside of Me the other day and OMG. This kid got a horrible fungal infection that was misdiagnosed for so long he died. There was fungal pores in the breeze,he breathed it in and got sick. Poor kid. There are diseases that affect your immune system where your body attacks himself, diseases that affect your skin,your organs,eyesight,senses. There are too many horrifying ones to pick just one.
Depression,you feel trapped be your own mind and body,all you want is to feel a bit better and always feel more down,death feels like the only way out,I just want all this pain to end! the pain inside is so much worse than pain on outside and you can't see depression,it all is bottled up inside,no insurance to pay for treatment,in stuck slowly hoping for death so this misery stops!