you cannot use the total population as the denominator for all disease incidence. There are risk pools which have significant large risk of developing certain diseases. for example, the risk of breast cancer among women,

By age 30 1 out of 2,212
By age 40 1 out of 235
By age 50 1 out of 54
By age 60 1 out of 23
By age 70 1 out of 14
By age 80 1 out of 10
Ever 1 out of 8