Patient Zero

Often involving claims that only by studying the patient zero can a cure be formulated, which is rubbish in most cases especially when no other options are even tried. This can make some sense if the virus or whatever has mutated since, so they'd have access to the "original" and thus supposedly generic cure for all mutations, which actually is rubbish too, having access to the original does not allow you to deduce how the rest have mutated and thus target them all.

A more sensible reason for finding patient zero is for finding where the outbreak began, the external cause for it if it has such, and a way to predict farther spreading. The original form of the contagion also does offer insight into the contagion (along with all other samples of it) even if it doesn't give some miracle cure mentioned above.

In smaller outbreaks where the spread of the contagion is limited, finding the patient zero also helps prevent farther spreading of it as you can quarantine one of its longest time spreaders and deduce other potential ones.