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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    Unless you're curious about any specific work in particular, rather than the trope in general, this is probably too broad to be answered here. – ApproachingDarknessFish Dec 2 '17 at 3:30
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    Not sure if this applies to scifi, but In the real world, finding patient zero is useful because it can help locate where the disease originated. Most diseases do not start in the human population, but are cultivated in another animal before spilling over into us. Finding patient zero means finding what species transmitted that disease to humans. Then treating it avoiding that species. Reinfection can occur depending on the disease. The seasonal flu is an example of this. It can change just enough that our immune system can't recognize it a second time, resulting in reinfection. – Sethen Dec 2 '17 at 4:14
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    It also tends to provide the 'original' form of the bug, before it mutated/adapted/etc. That is often essential to coming up with a cure for it. That said, unless you are defining a specific scifi universe, I believe this is a 'real world solutions' question. – K-H-W Dec 2 '17 at 4:41
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    The focus on the myth of patient zero comes from reporting on the early AIDS epidemic. Media depicted researchers searching for "patient zero" when the truth is there was never a patient zero. The idea came from a patient that researchers labeled "O" as in the letter. Researchers had been studying this person's behavior, and others, to prove AIDS was sexually transmitted. Media sensationalized and demonized "patient zero", who was really just another person in their study so much that now we focus on the idea in popular media. sciencedaily.com/releases/2016/10/161026132930.htm – Kai Dec 2 '17 at 13:27