The number of sick people who die from a specific disease. You calculate case fatality ratio by taking the number of people who have died from a disease and dividing it by the number of people who got that disease.

A CFR can change over space and time. Estimates of the case fatality rate for COVID-19 have varied from .7% to 4% or occasionally higher, depending on when the rate was sampled, how it was sampled and the quality of care those people received. Right now, the WHO says the global case fatality rate is about 3.4%. This number will continue changing as more people get infected and the disease spreads to countries with better and worse health care.