Frequently not always due to exposure to an infectious agent They can result from contamination of the environment (air, water, food, soil) by industrial pollutants Ex. Minamata disease in Japan from consumption of fish containing high concentrations of methyl mercury 1-The exposure from the same source may be prolonged-continuous, repeated or intermittent 2-No explosive rise in cases. Ex. Cases occur over more than one incubation period. Outbreak of respiratory illness, the Legionnaire disease in 1976 in USA, was a common source, continuous or repeated exposure, no evidence of secondary cases