As we move through the COVID-19 pandemic, you may have encountered two terms: morbidity and mortality. These related terms are commonly used in the field of epidemiology.

Morbidity and mortality describe the frequency and severity of specific illnesses or conditions.

There’s often confusion between morbidity and mortality, so this article will help explain the difference between them, along with several examples of each term.

Morbidity is the state of having a specific illness or condition. While morbidity can refer to an acute condition, such as a respiratory infection, it often refers to a condition that’s chronic (long-lasting). Some examples of common morbidities include:

You’ll often see morbidity data presented in two ways: incidence and prevalence. Let’s dive a little deeper into each of these.


Incidence refers to the occurrence of new cases of an illness or condition within a population over a defined period of time. It can be expressed as a proportion or a rate.


An incidence proportion can be used to estimate the risk for developing a specific condition during a given time period. It’s calculated by dividing the number of new cases during a specific period by the population at the start of the period.

For example, let’s say that 10 people became ill with food poisoning after eating undercooked chicken at a backyard barbecue. If 40 people attending the barbecue ate the chicken, the risk of food poisoning would be 25 percent.


The incident rate is the number of new cases of a disease within an at-risk population. This helps determine how quickly a disease is spreading. It’s often expressed in units of population, such as “per 100,000 people.”

For example, say you’re studying a population of 800,000 people at risk for developing hepatitis C. After 1 year, you find that 500 of those people have tested positive for the disease.

To calculate the incident rate, you’ll divide the 500 cases by the population of 800,000. You can then say that the incident rate of hepatitis C in this population is 0.000625, or 62.5 cases per 100,000 people per year.


Prevalence is the proportion of a population that has a condition or illness. Unlike incidence, it includes both new and existing cases. It can either be calculated at a specific point in time or over a specified period of time.

Prevalence is often expressed as a percentage. Population units, such as “per 100,000 people,” can also be used.

You may have also come across a term that’s related to morbidity. It’s called comorbidity. It means that you have more than one illness or condition (morbidity) at the same time.

Depending on the condition, some comorbidities may be more common than others. For example, according to the Centers for Disease Control and Prevention (CDC), arthritis is often associated with other comorbid conditions like obesity and heart disease.

Knowing whether you have comorbidities can be very important in a healthcare setting. That’s because they can make a difference in the diagnosis, treatment, and outlook of an illness.

COVID-19 is a good current example of this. If you have certain health conditions (comorbidities) and you also develop COVID-19, the risk of a serious illness increases. Some examples of these comorbidities include:

Knowing if someone who’s contracted COVID-19 also has one these conditions can make it easier for healthcare providers to develop an appropriate treatment plan in an effort to prevent severe illness.

Mortality refers to the number of deaths that have occurred due to a specific illness or condition.

Mortality is often expressed in the form of mortality rate. This is the number of deaths due to an illness divided by the total population at that time.

As with morbidity, mortality rate is often expressed in population units, typically as “per 100,000 people.” Let’s look at a simple example.

In 1 year, 50 heart attack deaths occurred within a population of 40,000. To determine mortality rate, you’d divide 50 by 40,000 and then multiply by 100,000. In this population, the mortality rate due to heart attack would be 125 per 100,000 people for that year.

According to the CDC, the top 10 causes of death in the United States for the year 2018 were:

  1. heart disease
  2. cancer
  3. unintentional injury
  4. chronic lower respiratory diseases
  5. stroke
  6. Alzheimer’s disease
  7. diabetes
  8. influenza and pneumonia
  9. kidney disease
  10. suicide

It’s worth noting that for the year 2020, COVID-19 will be a significant cause of mortality. A CDC report from late October predicts that the total mortality for COVID-19 in the United States will reach 250,000 to 266,000 individuals by mid-November.

If 2020 mortality rates from other causes remain similar to the numbers from 2018, COVID-19 will be the third-highest cause of death in 2020. This puts it just behind heart disease and cancer.

Generally speaking, the number of deaths remains relatively consistent in many populations from year to year. However, deaths can increase when events such as disease outbreaks, natural disasters, or wars occur.

Simply put, excess mortality is a comparison of the number of expected deaths versus the number of deaths that actually occurred.

COVID-19 is currently a cause of excess mortality throughout the world. A recent study found that deaths in the United States increased by 20 percent in the period spanning March through July of 2020. COVID-19 was the cause of 67 percent of these deaths.

The researchers theorize that the remaining excess deaths could be due to unrecognized COVID-19 or to other disruptions caused by the pandemic, such as interrupted access to regular healthcare.

Morbidity and mortality are two terms that are commonly used in epidemiology. While they’re related, they refer to different things. Morbidity and mortality are often expressed as a proportion or rate.

Morbidity is when you have a specific illness or condition. Some examples of common morbidities are heart disease, diabetes, and obesity. You can have more than one morbidity at a time. When this happens, it’s called comorbidity.

Mortality is the number of deaths due to a specific illness or condition. Common causes of mortality in the United States are heart disease, cancer, and unintentional injuries. For the year 2020, COVID-19 will also be a significant cause of mortality.