Dampness is one of the six climatic pathogenic factors (ie. a quality in the environment or climate which can cause disease). It is the predominant climatic factor in late summer/autumn, however, it can be encountered in other seasons.
Dampness pertains to Yin, so it is heavy. An attack by dampness will lead to symptoms such as heaviness of the head , limbs and body. Diseases caused by dampness also have the characteristics of stickiness and stagnation such as lots of sticky phlegm or mucus. They also tend to last a long time and are difficult to treat.