system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5; how we find “patient zero” when there is disease outbreak? (Like how they found who started the ebola outbreak in 2014)
gpt
First things first, Patient Zero was one guy, a specific patient who was “credited” with being the index case for HIV/AIDS. The general term for the first officially recorded case of a novel disease is “index case”, and it isn’t as big of a deal as movies often imply. It *is* helpful to trace the source of something like Ebola back to an animal reservoir, and from an epidemiological perspective you want to be sure that you’re at the end of the chain of infection. 

In reality though you rarely know for sure who the index patient was overall or for a given outbreak, so it ends up being a bit de facto “who did health authorities notice first?” The Index Case is more of a result for how contact tracing and other epidemiological techniques work in practice than some sort of holy grail.