Often the death rate for a disease is fully known only for a group where the disease has been well studied.  For example, the 3711 passengers on the Diamond Princess cruise ship are, to date, the most fully studied coronavirus population.  All passengers were tested and tracked by health authorities, and the death rate was approximately 1%. However, the age profile of the Diamond Princess is different than that of most countries – it skews heavily to older people.  The disease outcomes on the Diamond Princess can be applied to other populations by adjusting them to the target population:

Standardized death rate =  sum(Pk)(dk)/sum(Pk)

Where 

the population is divided into k age groups
Pk is the target population proportion in the kth age group
dk is the death rate in the kth age group of the study population

In this way, the results from the study can be applied to other populations (e.g. countries) with different age distributions.  Of course, this assumes the treatment and other conditions pertaining to the disease are the same in the study population and the target population.