The dissemination of plant parasites is favored by many circumstances not always obvious, whence an air of mystery regarding epidemics was easily created in earlier times.
The origin of these two epidemics was obscure.
Endemic diseases are unknown and epidemics are rare.
Yellow fever epidemics are common on the Campeche coast, and sometimes appear at Progreso and Merida.
The epidemic nature of wheat-rust was known to Aristotle about 350 B.C., and the Greeks and Romans knew these epidemics well, their philosophers having shrewd speculations as to causes, while the people held characteristic superstitions regarding them, which found vent in the dedication of special festivals and deities to the pests.