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In addition to the six pathogenic factors, there is another category
known as pestilential factors which is the source of epidemic disease.
Its nature is similar to pathogenic heat and summer heat, but more
pernicious and more fierce in pathogenicity; it is usually accompanied
by pathogenic damp. Pestilential diseases are epidemical and dangerous,
with rapid drastic changes as seen in small pox, plague, cholera, and
most acute contagious diseases.