miasma

瘴 is composed of the radical 疒 (sickness) and 章 (seal). 疒 denotes illness, while 章 depicts the action of carving a seal with a cutting tool, carrying the sense of being prominent or clearly visible. Because the dense, humid miasma in mountain forests is visibly apparent and causes people to fall ill, 瘴 was formed to represent this disease-inducing tropical mist or miasma.