How did the first ever person to get a disease, actually get it if no one passed it on?

Question: How did the first ever person to get a disease, actually get it if no one passed it on?
- Keywords:
  history,
  infection
Asked by andrei to PB on 21 Jun 2013.
- Peter Balfe answered on 21 Jun 2013:
  
  Fantastic question! There is a theory, which I subscribe to, that ultimately all infectious diseases are zoonotic, that is they came from an animal and that’s how the first person got it. Some reached humans millions of years ago, some more recently. For example, Syphilis is thought to be ~1,000 years old and probably came from camelids (camels, llamas etc.), which carry an almost identical bacteria.

Question: How did the first ever person to get a disease, actually get it if no one passed it on?