I was on an epic adventure to examine a fortress that I lost due to a mysterious crashing bug, when I came across a prisoner in a pitiable state. A towering capybara twisted into humanoid form composed of green zircon. It undulates rythmically. Beware its poisonous gas! It begged me to take it with me, away from its human captor. I considered how it would make a lovely match with the jade dragon kept in the fortress basement, and of course asked it to join.
Perfect companionship on a voyage fraught with danger, as long as I stayed far enough away to avoid that gas. Easy! Or so I thought.
Since you're reading this, you know what happened. One night out in the wilderness, I woke up coughing, boiling extract all around. I dove into the nearby pond in an effort to cleanse myself, but it may already have been too late. From then on, as we marched, I kept a close eye on my health, afraid of catching a glimpse of Symptoms. I even recruited a human Doctor and Surgeon to aid me in my quest. As the days wore on, I started to believe that I got away clean.
The second time, however, I was less fortunate. In the chaos of fighting off a large wolf-pack, my zircon "friend" breathed gas on me. Without cleansing water in view, I was forced to walk for miles before having a chance to clean myself, and this time I knew for sure I was a goner.
Still, while walking all day and sleeping all night, symptoms started creeping up only slowly. I first noticed a slight numbing sensation in my extremities. By the time we reached the fortress we were looking for, my thinking was fuzzy and I started smelling an odd stench. The Doctor examined me in the old hospital and found that my central nervous system appeared to show signs of rot. In the enclosed, dead air of the abandoned fortress, the disease started to progress more quickly. By the time I made it to the fortress' legendary clear glass dining room, blood had started to swell my entire body, fluid filling every organ and every limb. The brain-rot slowed my movements, and then I could not stand. As the cells in my spine underwent a cascade of apoptosis, my hands would no longer grasp the spear and shield that had protected my life from my enemies.
My enemies. Those who would harm me. Because of my friend, now they never ever would.
TL;DR: using fast travel and sleep seemed to have postponed syndrome symptoms. The demon's extract boiled away in no time at all, so I don't think I actually accomplished anything by diving into a pool that first time. The second time, I was able to travel for a week before noticing anything bad. Some of my companions showed symptoms eventually, but none died from it afaik. On what sort of time-scale do diseases work, anyway?