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3. Generate Hypotheses

Developing a hypothesis regarding the cause of the outbreak is often challenging and is a crucial step in the outbreak investigation.

Many pathogens that cause waterborne diseases can also be transmitted by contaminated food or by contact with an infected person or animal. When looking for the source of the illness, investigators first need to decide on the likely mode(s) of transmission. The identified pathogen, where ill persons live, or the age of the patients may suggest a particular mode of transmission and could help identify a specific source. Hypothesis generation should be considered an iterative process in which possible explanations are continually refined or refuted.

When exposure to water is suspected as the source of contamination, public health officials interview ill cases to determine water exposures in the days or weeks prior to onset of illness. These interviews are called “hypothesis-generating interviews.”  Interviews can either use a standardized questionnaire (e.g., “shotgun” questionnaire), or they can be open-ended. Standardized interviews include a set of questions used by public health officials to interview ill people during outbreak investigations.  Open-ended interviews are not standardized and do not provide concrete exposures for analysis. Interviews will focus on activities and experiences that occurred during the pathogen’s incubation period—the time it takes to get sick after exposure to the contaminated water. A table of common waterborne pathogens and their incubation period is listed in the Appendices.

Based on all the information gathered, the investigators make a hypothesis about the likely source of the outbreak. If they are not able to develop a hypothesis, investigators can return to intensive, open-ended interviews or utilize a different set of standardized questions to develop clues to the outbreak source. Clues to the outbreak source might come from ill persons with few exposure opportunities or from interviewing cohorts (e.g., family groups or sports teams) within the larger outbreak population.

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