The term comorbidity was coined in the 1970s by A.R. Feinstein. He gave the example of people suffering from rheumatic fever also usually suffered from multiple other diseases (source).

Comorbidity can be defined as the existence of two or more disorders or illnesses occurring in the same person, either simultaneously or sequentially. It is associated with worsening health outcomes, more complex clinical management, and increased health care costs (source).

Primary Disease Plus Additional Disease

Typically, the primary disease is considered the disease for which the patient is seeking medical care, and the additional disease(s) are the comorbidities. 

It is often difficult to determine which disease is the primary disease, and which are the comorbidities.  Patients diagnosed with obesity also often have sleep apnea, high blood pressure, type II diabetes, and high cholesterol.  There may be no way to tell if the patient’s obesity lead to their other diseases, or if one of the diseases led to their obesity.

The idea of “burden of disease” can be linked to the patient’s psychological ability to deal with additional diseases, the severity of the comorbidities, and the patient’s socioeconomic environment and ability to receive medical care for their increased disease burden.

Simultaneous, Independent Diseases

This type of comorbidity may be:

  • Medical (arthritis and asthma in the same patient)
  • Behavioral (asthma and ADHD in the same patient)
  • Psychological (multiple sclerosis and major depressive disorder)

These diseases may have no medical association, and are simply present at the same time due to chance. 

They may also have associated risk factors: smoking is a risk factor many diseases and causes harm to every major organ system (source). A person who smokes is at risk for developing coronary heart disease, stroke, and cancer (source). A person may have both coronary heart disease and lung cancer, both of which may be traced back to a history of smoking tobacco.

Pathologically Linked Diseases

More commonly, comorbidities can be pathologically related to each other (and is often confused with disease complications).  Examples include:

  • ADHD with anxiety and other psychological disorders (source)
  • Alzheimers disease with diabetes, obesity, and high cholesterol (source)
  • Arthritis with obesity, diabetes, and heart disease (source)
  • Diabetes and high blood pressure, obesity, high cholesterol, liver disease (source)

Misdiagnosis of Main Disease

The presence of more than one disease may be due to a third, distinct, disease.  A diagnosis of metabolic syndrome requires the presence of three or more of the following risk factors: obesity, high blood pressure, diabetes, high triglycerides, and/or low HDL cholesterol.  These so-called comorbidities may simply be symptoms of a broader, overarching disease.

Dyads and Triads

Comorbidities commonly appear as dyads (two diseases) or triads (three diseases), but patients can have four or more co-morbid diseases at once.  Medicare has defined 210 dyads, and 1,330 triads in their research of chronic disease.