Why Individual Behavior Is Key to the Spread of Viruses Such as Covid-19

Diseases are spread due to the behavior of people. One may have millions of differently behaving people. For simplicity, humankind is only considering the average behavior. However, this simplification can lead to results tremendously different from the exact solution in at least some applications when non-linear differential equations are used. In this letter, we prove that the mistake is very big for the spread of infections like Covid-19. Ten percent ignoring the rules can almost ruin the extremely careful behavior of the remaining ninety percent. This is totally different to most business situations where considering eighty percent is sufficient (“80-20-rule”). This may explain why the spread of the Covid-19 pandemic and the impact of measures against it are hard to predict for decision makers.