Comorbidity

# Background
In medicine and in psychiatry,  comorbidity (literally "additional morbidity") is either
- The presence of one or more disorders (or diseases) in addition to a primary disease or disorder; or
- The effect of such additional disorders or diseases.

# Comorbidity in medicine
In medicine, comorbidity describes the effect of all other diseases an individual patient might have other than the primary disease of interest. There is currently no accepted way to quantify such comorbidity.
Many tests attempt to standardize the weight or value of comorbid conditions, whether they are secondary or tertiary illnesses. Each test attempts to consolidate each individual comorbid condition into a single, predictive variable that measures mortality or other outcomes. Researchers have "validated" such tests because of their predictive value, but no one test is as yet recognized as a  standard.
The term "comorbid" currently has two definitions: 1) to indicate a medical condition existing simultaneously but independently with another condition in a patient (this is the older and more "correct" definition) 2) to indicate a medical condition in a patient that causes, is caused by, or is otherwise related to another condition in the same patient (this is a newer, nonstandard definition and less well-accepted).

## Charlson index
The Charlson co-morbidity index (CCI) predicts the 1 year mortality for a patient who may have a range of co-morbid conditions such as heart disease, AIDS, or cancer (a total of 22 conditions). Each condition is assigned with a score of 1,2,3 or 6 depending on the risk of dying associated with this condition. Then the scores are summed up and given a total score which predicts mortality. 
For a physician, it's helpful in knowing how aggressively to treat a condition. e.g. A patient may have cancer, but also heart disease and diabetes so severe that the costs and risks of the treatment outweigh the short term benefit from treatment of the cancer.
The CCI was recently rated as low.
The CCI may not perform as well as the Apache (area under curve 0.67 vs 0.87).

## Diagnosis-related group
Patients who are more seriously ill tend to require more hospital resources than patients who are less seriously ill, even though they are admitted to the hospital for the same reason. Recognizing this, the diagnosis-related group (DRG) manual splits certain DRGs based on the presence of secondary diagnoses for specific complications or comorbidities (CC).

# Comorbidity in mental health
In psychiatry, psychology and mental health counseling comorbidity refers to the presence of more than one diagnosis occurring in an individual at the same time. In psychiatry, comorbidity does not necessarily imply the presence of multiple diseases, but instead can reflect our current inability to supply a single diagnosis that accounts for all symptoms. On the DSM Axis I, Major Depressive Disorder is a very common comorbid disorder.  The Axis II personality disorders are often criticized because their comorbidity rates are excessively high, approaching 60% in some cases, indicating to critics the possibility that these categories of mental illness are too imprecisely distinguished to be usefully valid for diagnostic purposes and, thus, for deciding how treatment resources should be allocated.
Comorbidity is also found to be high in drug addicts, both physiologically and psychologically.