Morbidity refers to the unhealthy state of an individual, while mortality refers to the state of being mortal. Both concepts can be applied at the individual level or across a population. For example, a morbidity rate looks at the incidence of a disease across a population and/or geographic location during a single year. Mortality rate is the rate of death in a population. The two are often used together to calculate the prevalence of a disease — e.g., measles — and how likely that disease is to be deadly, particularly for certain demographics.
What is Morbidity?
The word morbid is related to sickness and disease. As a concept, morbidity can be applied to an individual (e.g., someone with diabetes) or to a population in the form of a morbidity rate (e.g., the incidence of seasonal flu). There is also comorbidity, which refers to two or more illnesses affecting an individual at the same time. For example, gout is often comorbid with diabetes.
Morbidity rates vary depending on the disease in question. Some diseases are highly contagious, while others are not. Similarly, some diseases are more likely to affect one demographic than another. Morbidity rates help doctors, nurses, and scientists to calculate risks and make recommendations for personal and public health matters accordingly.
What is Mortality?
All humans are mortal, subject to death. A "crude death rate" — the total number of deaths in a year, per 1,000 individuals — can be used to see how many people are dying in the world. This rate is often paired with those used to calculate the number of people being born (e.g., crude birth rate), so as to estimate the total living human population on the planet.
The rate at which humans die varies tremendously by geographic location, wealth, incidence of illness (morbidity), age, etc. For this reason, there are several different kinds of mortality rates, such as the maternal mortality rate (number of deaths of mothers due to child bearing), infant mortality rate (number of deaths of children under one year of age), or age-specific mortality rate (total number of deaths of a particular age group). Using all of these different mortality rates paints a more accurate picture about global health and well-being.
Units of Measurement
Morbidity can be scored to determine disease severity and the need for medical intervention. It can also be predicted to determine disease risk and compare patient illness and outcomes between hospitals. Standardized disease classification systems, such as the APACHE II, SAPS II, and Glasgow Coma Scale, enable doctors around the world to offer similar, science-based care to their patients.
While mortality is usually expressed as the number of deaths per 1,000 individuals in a year (i.e., mortality rate), mortality may also be scored or predicted. For example, the SAPS III, PIM2, and SOFA scoring systems offer a way to realistically predict the mortality of a person in intensive care. Scoring and predicting mortality is a good way for hospitals to improve conditions and treatment from year to year.
Gathering reliable statistical data for morbidity and mortality rates can prove difficult, particularly in less developed nations, where reporting standards may be poor. However, it is worthwhile to gather statistics related to morbidity and mortality, as doing so can lead to improving life quality around the globe.
According to a 2009 WHO report, 6 out of every 10 deaths in the world "are due to noncommunicable conditions; 3 to communicable, reproductive or nutritional conditions; and 1 to injuries." In developing nations, mortality is often linked to infectious diseases and pregnancy/childbirth. In more developed places, cancer and cardiovascular diseases — illnesses largely affecting older populations — are more common causes of death.
It is possible for a disease that is widespread (high morbidity rate) to have a low mortality rate, or vice versa, and these rates may change over time as environmental changes or or medical advancements occur. For example, HIV/AIDS spread rapidly during the 1980s and 1990s, and had a very high mortality rate, but, today, in places where good HIV prevention education and medical care are available, both the morbidity rate and mortality rate for HIV infection have decreased significantly. In contrast, in poorer areas of the world, the spread of HIV is still of great concern, and the mortality rate for the disease remains high in places where medicine is scarce.
The United Nations (UN), World Health Organization (WHO), and Center for Disease Control and Prevention (CDC) are just a few of the organizations that frequently compile data pertaining to diseases, mortality, causes of death, and mortality rates. Most all of this data can be viewed, for free, online.
There are also publications dedicated to analyzing changes in morbidity and mortality, specifically. For instance, the CDC in the U.S. publishes a weekly morbidity and mortality report (MMWR); Europe, in conjunction with WHO, keeps a European hospital morbidity database (EMDB); and morbidity data for Australia can be found in its national hospital morbidity database (NHMD).
A Human Mortality Database was developed in the late 1990s / early 2000s by the University of California, Berkeley's Department of Demography and Germany's Max Planck Institute for Demographic Research. This open database provides mortality statistics and other population data for 37 countries.