World leaders and public health experts are poised to spend the coming months or years obsessed with a variable known as R0.

Pronounced “R-naught,” it represents the number of new infections estimated to stem from a single case.

In other words, if R0 is 2.5, then one person with the disease is expected to infect, on average, 2.5 others.

An R0 below 1 suggests that the number of cases is shrinking, possibly allowing societies to open back up. An R0 above 1 indicates that the number of cases is growing, perhaps necessitating renewed lockdowns or other measures.