What is the difference between morbidity and mortality?

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The other day, someone in our epidemiology discussion asked what’s the difference between morbidity and mortality, and it really made me think back to what I had read in a CDC bulletin. Morbidity is about illness and health conditions people live with, while mortality is about death rates. Knowing both gives a better understanding of public health.

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Barry Mosley 2025-06-07T09:11:22+00:00 1 Answer 2 views
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  1. fleming
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    2025-06-07T09:13:30+00:00
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    Morbidity

    Definition:

    Morbidity refers to the state of being diseased or unhealthy within a population. It encompasses sickness, illness, disability, or any deviation from a state of physiological or psychological well-being.

    Focus:

    Morbidity focuses on the burden of illness and non-fatal health outcomes.

    Measures:

    • Incidence Rate: The rate at which new cases of a disease occur in a population over time (measures risk of developing the disease).
    • Prevalence Rate: The proportion of a population that has a specific disease at a point in time or during a period (measures burden of existing disease).
    • Attack Rate: A specific type of incidence rate used during outbreaks, representing the proportion of an exposed population that becomes ill.
    • Disability-Adjusted Life Years (DALYs): A composite measure combining years of life lost due to premature mortality and years lived with disability.
    • Quality-Adjusted Life Years (QALYs): A measure of health outcome that adjusts life expectancy for quality of life.
    • Rates of specific conditions (e.g., asthma prevalence, diabetes incidence).

    Information Provided:

    • The frequency and distribution of diseases in a population.
    • The types of illnesses affecting a community.
    • The level of disability and functional impairment caused by health conditions.
    • Risk factors associated with developing diseases.
    • The need for healthcare services and prevention programs.

    Examples:

    • The prevalence of arthritis in adults over 65.
    • The incidence of influenza during the winter season.
    • The number of hospitalizations due to asthma attacks.
    • The rate of non-fatal injuries from car accidents.

    Mortality

    Definition:

    Mortality refers to the state of being subject to death, or more specifically, the occurrence of death within a population.

    Focus:

    Mortality focuses on fatal health outcomes.

    Measures:

    • Crude Death Rate: The total number of deaths in a population during a specific period, divided by the total population (usually per 1000 people).
    • Age-Specific Death Rate: The number of deaths in a specific age group divided by the population in that age group.
    • Cause-Specific Death Rate: The number of deaths due to a specific cause divided by the total population.
    • Infant Mortality Rate (IMR): The number of deaths of infants under one year of age per 1000 live births.
    • Maternal Mortality Ratio (MMR): The number of maternal deaths per 100,000 live births.
    • Case Fatality Rate (CFR): The proportion of individuals with a specific disease who die from that disease during a specific period.
    • Years of Potential Life Lost (YPLL): A measure of premature mortality.

    Information Provided:

    • The overall death rate in a population.
    • The leading causes of death.
    • Differences in death rates among various demographic groups (age, sex, race).
    • The lethality or severity of specific diseases (Case Fatality Rate).
    • Trends in life expectancy and premature death.
    • The effectiveness of interventions aimed at reducing deaths.

    Examples:

    • The crude death rate for a country in a given year.
    • The infant mortality rate.
    • The number of deaths attributed to heart disease.
    • The case fatality rate of Ebola virus disease during an outbreak.

    Key Differences Between Morbidity and Mortality

    Feature Morbidity Mortality
    Definition State of being diseased/unhealthy Occurrence of death
    Focus Illness, sickness, disability Death, fatal outcomes
    Outcome Non-fatal health conditions Fatal health conditions
    Measures Incidence, prevalence, DALYs, attack rate Death rates (crude, specific), IMR, CFR
    Information Burden of illness, risk factors, healthcare needs Causes of death, severity, life expectancy

    Relationship:

    Morbidity and mortality are often related. High morbidity from a severe disease can lead to high mortality. However, a disease can have high morbidity but low mortality (e.g., arthritis, common cold), causing significant illness and disability but few deaths. Conversely, some conditions might have relatively low morbidity (few recognized cases) but high mortality (high case fatality rate, e.g., rabies). Studying both provides a more complete picture of a population’s health status than either measure alone.

    Source: Dictionary of Epidemiology (IJE); Centers for Disease Control and Prevention (CDC) Morbidity and Mortality Weekly Report (MMWR)

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