(1):

(n.) A morbid condition produced by exhalations from decaying vegetable matter in contact with moisture, giving rise to fever and ague and many other symptoms characterized by their tendency to recur at definite and usually uniform intervals.

(2):

(n.) Air infected with some noxious substance capable of engendering disease; esp., an unhealthy exhalation from certain soils, as marshy or wet lands, producing fevers; miasma.