In the simulation you did, the disease was transmitted every time an infected person met an uninfected person who was not immune. in the real world, these people may not have become sick. why not? state as many reasons as you can.

This phenomenon is the healthy carrying (healthy carrier)

It is generally a person who can transmit a disease to another without being himself sick. One can be a healthy carrier of an infectious disease, that is to say, harbor germs that can be dangerous for others.

For example, following an old infection apparently cured.

The disease may not be manifested because of the person's immune system, so that the infectious agent transform into a kyst and hides in the different tissues immune to the immune system. Or the infectious agent does not stay long in the body, it will just have time to pass to another agent.

Another reason may explain the healthy carriage that is vaccination.