I am trying to think of ideas for the deadliest disease that kills humans, not other animals. Of course a disease that had no symptoms and started killing people immediately would be the best, but I am looking for a very deadly disease with at least one noticeable symptom. What would the disease and its symptom(s) be?
Disease that causes paralysis like a more powerful botulism. One-symptom that also mean your heart get paralyzed or the muscles that allow you to breath.
Itchy feet syndrome- you always feel that you don't fit in so you have to travel. The greatest mechanism for transmission is simple migration. Ebola is contagious without symptoms. Give your illness air-borne transmission and itchy feet syndrome and finally death and it would be the worst pandemic of all time.
Given that death is the ultimate symptom, you already have your one symptom right there...
But you probably mean one symptom apart from death. Such a symptom should ideally be one that makes the disease more infectious. So coughing, skin rashes, maybe cysts or boils that break and spread the disease agent on touching them. Things like that.
The more innocious the better of course, ideally that symptom should be indistinguishable from other, pretty harmless, conditions in your universe. So if there's regular dust clouds that have people sneeze, sneezing is not seen as a sign that you're infected with that disease, allowing the disease to spread unnoticed until the victims start dropping like flies.