Morbidity: a measure of all sickness or ill-health. Usually expressed as proportion (ratio) of a standard unit of the population (usually 10,000 or 100,000).

Mortality: ratio of the number of deaths to a given unit of population.

Incidence: (Number of new cases of a disease
per unit of time) : (Total population at risk)

Prevalence: (Number of persons with a disease
at a given point in time) : (Total population)

Most of the measures of health and disease are expressed in the form of ratios. A ratio is a relative number expressing the magnitude of one occurrence in terms of another. Usually it is used in a specific sense where the numerator and denominator are two separate and distinct quantities. Neither is included in the other. A ratio does not depend on the size of the number, but on their relation to each other; i.e.,6 to 4, 75 to 50 and 10929 to 7286 may al be described by their ratio of 3:2 . Proportions are a type of ratio in which the numerator is included in the denominator, i.e. a/ (a+b). Some epidemiological expressions, which are in fact proportions, are loosely described as rates, e.g. proportion of fetal deaths= (number of fetal deaths /no. of conceptions) is usually referred to as the fetal death rate. It is in fact the proportion or relative frequency of fetal deaths among all conceptions and can be used as an estimate of the probability of this event.