Biases from the health system
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Even if all cases of disease diagnosed at clinics are reported, for most diseases, most people with illness never come to health facilities for treatment. As a result, these cases cannot be reported in a surveillance system based on health facility reporting. Incidence rates calculated from only reported cases may greatly underestimate the true incidence rate in the population.
Seeking health care
In Goma, Zaire, a survey was done among Rwandan refugees in August 1994 in the midst of a huge cholera and dysentery outbreak. It found that less than 50% of people who died of cholera had sought medical care at a health facility. As a result, reliance on data from health facilities would have underestimated not only the incidence rate of cholera, but also the death rate from cholera, by more than one-half.