What does evolutionary methods mean in terms of tracing origins of diseases? What does this look like? Does this mean they're testing diseases on animals or are they extracting viral DNA to do analysis and comparing to other viral DNA? Confuzzed.

I remember reading "The Hot Zone" by Richard Preston several years ago about all of the symptoms of Ebola, and how they scanned every millimeter of a bat cave to find the origin of the virus, but ended up with nothing. Crazy to think a tiny little virus in such isolation has so much potential power.