Patients with one inflammatory diagnosis are at higher risk for developing a second. For example, one study found that, in healthy European middle-aged men, symptoms of depression were associated with a higher risk of coronary heart disease in the short-term and stroke over the long-term. The simultaneous presence of two or more chronic diseases or conditions in a patient is referred to as comorbidity.

The overlap between the symptoms associated with comorbid conditions is consistent with the process of successive infection. No two people exposed to pathogens over time ever acquire the exact same mix of species in their microbiome. Therefore, no two individuals will ever develop an identical disease presentation, and the symptoms of patients with similar diagnoses can be expected to overlap and fluctuate with time.

The figure to the right shows the extent to which patients with one inflammatory disease often suffer from other related inflammatory conditions. Each “spoke” on the wheel connects two separate diseases that have been shown to occur together in a published study.

This overlap between disease states suggests that different inflammatory diagnoses are best studied collectively. It is commonly believed that one inflammatory condition causes a second, although the mechanisms behind these assumptions remain unclear. For example, obesity is generally believed to cause diabetes. Yet, increasing evidence shows that the microbiome is significantly altered in patients with both conditions. It may be more likely, then, that both obesity and diabetes gradually occur together because of successive infection or a common underlying pathogenesis.

In the same vein, patients suffering from physical inflammatory conditions are at greater risk for developing neurological dysfunction and vice versa. This suggests that neurological and autoimmune diagnoses, which are currently separated into different medical specialties, may also arise from similar underlying infectious processes. Indeed, dysbiosis of the microbiome has been documented in patients with a range of neurological conditions including multiple sclerosis, autism and obsessive-compulsive disorder.