system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How are scientists able to track the birthplace and movements of diseases like AIDs or the Black Death, even after people are long dead?
gpt
Through studying the remains of the dead, written records and genetic studies.

When it comes to remains, in case of ancient plagues this is usually restricted to the bones, usually found in mass graves indicating people had no time nor want to properly bury their dead.  With more recent diseases, scientists can often study older preserved samples of blood or tissue, this way they found cases of people who had HIV/AIDS as far back as the 50s for example.

Then there are the written records - even if people didn't understand/recognize the disease, description of sympts are telling. We know for example from records of social workers in the 70s about an illness they then dubbed junkie flu/pneumonia which pretty well matches AIDS. With the black death, mentions of symptoms like thee emblematic buboes, among others in historical records is how historians can track where the plague appeared.

Finally, modern genetic studies allow modelling how the germ evolved and spread. Rates of mutation are thought to be constant, and so researchers could determine approximately when and where did HIV evolve from it's predecessor SIV.

All in all, historical research of diseases is a fascinating field that requires a multidisciplinary approach from a variety of experts pooling their knowledge together.