In pretty much every sci-fi that deals with epidemics or mass illness in some way they are looking for patient Zero. My question then is: Why is finding "patient zero" in an epidemic so important? Also does it happen in epidemiology that some people are infected and reinfected multiple times from different sources, making it really difficult to trace? Like Farmer Bob got Tuberculosis from his cows, and again from the contaminated ground, and then from a friend, and then another new strain at the hospital? How would you sort through all that?
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