How do you know if a disease has spread?

Question: How do you know if a disease has spread?
- Keywords:
  disease,
  infection
Asked by anon-244170 to Pan-ngum on 17 Mar 2020.
- Pan-ngum Wirichada answered on 17 Mar 2020:
  
  Without an active surveillance or being in a small and closed system, it is not easy to know when a disease has spread. We often realize when we see increasing in number of infections. If we detect it early enough then we may be able to slow or stop it spreading but this isn’t easy to know.

Question: How do you know if a disease has spread?