Comorbidity can be defined as the existence of two or more disorders or illnesses occurring in the same person, either simultaneously or sequentially.  Typically, the primary disease is considered the disease for which the patient is seeking medical care, and the additional disease(s) are the comorbidities.  Comorbidity is also used to convey the idea of “burden of disease.”  This 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, but independent comorbid diseases may be medical (Arthritis in Asthma patients), behavioral (a patient with both Asthma and ADHD), or psychological (Major Depressive Disorder is a very common comorbid 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 for COPD, and excessive alcohol use may lead to liver cirrhosis.  These diseases are not pathologically linked, but the risk factors are associated with one another.

More commonly, comorbidities can be pathologically related to each other.  This type of comorbidity can often be confused with disease complications.  For example, a patient with diabetes may eventually go blind: the patient may have both diseases at the same time, but the blindness was most likely caused by the diabetic patient’s hyperglycemia.

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.

Additionally, 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.

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 171 dyads, and 969 triads in their research of chronic disease.

Comorbidity is associated with worsening health outcomes, more complex clinical management, and increased health care costs.

Read more about the research into comorbidity here:

Please refer to the following links (to excel files) for the most recent data: