From my experience, sniffle sniffle, cough, cough, areas in which unsanitary drug use practices abound, are more likely to create a general feeling of cruminess i.e. runny nose, feeling run-down and fatigued, coughing, respiratory congestion, and body aches. By unsanitary drug use practices, I am referring to a group of stoners sharing Humboldt Kind out of shared bongs, joints, and pipes with no regard given to the various infections that could be transmitted through the said devices. Humboldt Sludge could of course exist anywhere that people sit around and get high, but this name is unique in that it puts a specific label on a general phenomena, that of stoners sharing germs. The moist and spore-filled coastal climate of Humboldt County provides an additional breeding ground for the development of Humboldt Sludge. In addition, we can't discount the possible effects of smoke on the immune system.
The exact mechanism of this ailment is far too complex to analyze here, involving the interplay of virus, bacteria, and fungi, but the treatment is simple: bed rest, not sharing smoking devices, and plenty of fresh fruits and vegetables, and perhaps some supplements of Vitamin C and Zinc.
It could of course be argued that the Humboldt Sludge phenomena doesn't exist and is merely a subjective experience of being a lazy fatigued stoner. I would argue that there is no way to tell for certain if it objectively exists, because a scientific experiment duplicating the myriad of factors in Humboldt County (or an analogous area) stoners' lives, such as proximity of living quarters, diet, other drug or alcohol use, and other lifestyle choices, would be near impossible. So the best I can say is, see for yourself.