One gauge of a virus is how contagious it is. The flu has an infectiousness measure (or R0) of only about 1.5, meaning that each sick person infects on average 1.5 others. In contrast, Covid-19 without social distancing appears to have an R0 of perhaps 2.5.
A second gauge of a virus is how often infected people must be hospitalized. With the seasonal flu that’s roughly 1 percent; with the coronavirus, estimates range from 5 percent to 20 percent.
A higher R0 and higher hospitalization rate conspire to wreak havoc.
A single person with the flu can result in the infections of 386 other people over two months, and a handful would be hospitalized. But in that same period one Covid-19 patient could lead to the infections of 99,000 people, of whom nearly 20,000 might need to be hospitalized.
A third measure is lethality, the “case fatality rate,” or percentage of people who contract an illness who eventually die of it. For the flu, this is about 0.1 percent. For Covid-19, there are enormous uncertainties but even in optimal circumstances it may be 10 times greater, roughly 1 percent – although it has been
much higher than that in countries like Italy with older populations and overburdened hospitals.