healthy
infected, not infectious yet
infectious
recovered (immune)
vaccinated
died
· house = staying home
Everyone over time
Each day stacked to 100% of people. The red band at the bottom is how many are infectious at once: the "curve" that measures try to flatten.
How it works
Every person is in one of five states. A healthy person who stands close to an infectious one can catch the virus. First they are infected but cannot pass it on yet (the incubation). Then they are infectious for some days, and after that they either recover and are immune for a while, or die. The spread number R is how many people one infectious person passes it on to, on average. Above 1 the outbreak grows; below 1 it dies out. Vaccines, masks, quarantine and lockdown all work by pushing R below 1.