The flu. The perfect illness. Easily transmittable/spread widely. Changes every year- we can't nail it down. The flu is two Gygaxian 20-sided dice rolling numbers. 99% of the time the result is nominal but every 1% they are exceptional, such as the Spanish Flu of 1918. One person can infect 100 more, from there is it becomes math. Every once in a while a particularly strong mutation occurs producing a deadly flu. Last one was a century ago. Imagine us being outdone by a virus- what would that say of humanity?