A town of people who live, work and go to school together. Pick a virus, decide who gets vaccinated and how fast, and watch what happens.
One run can be lucky or unlucky. This runs the same town several times with your settings and several times with no vaccination at all, and compares the averages.
Changing the virus, the town, the first cases or "vaccinated before the outbreak" starts again from day 0. Everything else takes effect while it runs.