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An epidemiological investigation was carried out in order to identify the source of infection in an epidemic of typhoid fever, which occurred from February 20 and ended by March 11, 1983 in Wcity of Korea, with population about 140,000. The analysis of confirmed cases and sample survey on the groups of patient’s family and healthy family led the conclusion that the source of infection was the tap water supplied by the second water plant, which supplies 90% of the water for the city, based on the following findings: (1) The incidence rate per 10,000 population was as low as 8.7 involving all ages with the highest rate among children 5~9 years of age and relatively high occurrence among housewives, which is a typical characteristic of water-born epidemic of the disease. (2) The frequency distribution of the cases by the date of onset showed a single peak positively skewed, all cases occurred within the range of one incubation period suggesting one single exposure to the agent. (3) The geographical distribution of the cases showed a strong association with the water supply status from the second water supply plant. (4) The result of the sample survey on patient's family and healthy family showed difference only in source of drinking water; there was no difference between two groups in food markets they utilized and the foods they have eaten during the Lunar New Year Holiday, when was estimated to be the time of exposure. (5) Most of the cases surveyed by interview had a peculiar characteristics such as young children, housewives, and persons with habit enjoying raw water and foods, who all had vulnerability to the exposure and infection for low dose agent.