About a quarter of the American population suffers from a mood disorder.

Mood disorders are best characterized as the instability of one's moods or emotions, so much so that it interferes with one's normal functions. These are often actual psychiatric disorders, like bipolar disorder or depression, but can occur as short-term mood swings, like seasonal affective disorder, which stems from the change in seasons between warmer and colder months. Mood disorders affect quite a few people; some of them are hereditary, while others are environment-based.

New research indicates that a history of any kind of infection that required hospitalization adds to one's likelihood of developing one of these mood disorders.

In a study of three million people, 62 percent were more likely to develop a mood disorder after hospitalization for an infection, while 45 percent with an infection related to an autoimmune disorder were more likely to develop a mood disorder. Out of three million, 91,637 people had developed a mood disorder after hospitalization for an infection.

Infections are related to a minor or temporary failure of one's immune system to detect and kill invading disease-causing agents. What's more, they can get serious if they occur in major organs like the spleen, kidney, or heart, but often can be treated with antibiotics. And in the cases where they get serious enough to warrant a hospital visit, they are implicated in the development of mood disorders.

An autoimmune disease is a disease where one's immunity is altered from birth. People who suffer from autoimmune disorders have immune cells that recognize and mark their own host cells as foreign and try to destroy them. This too can lead to serious infections, as the immune system is detecting the body's normal cells as attackers. Examples of autoimmune diseases include psoriasis, which attacks skin cells, and rheumatoid arthritis, which attacks tissues at the joints.

Often, the method that the body will use to eradicate its invader is the development of a fever and the local attack of the foreigners by particular immune cells, which leads to a state called inflammation. The fever and direct attack by certain cells will kill off the invader by making it unable to infect other cells. However, if the fever and inflammation persist for too long, the person's health could be harmed, leading to hospitalization.

The strength of this immune attack causing hospitalization and creating the likelihood of a mood disorder indicates a clear link between disease spread and how mood disorders stem from it. This is the first time a clear correlation has been drawn between infection and the risk of developing mood disorders.

Anyone can contract an infection to any part of their body. This finding indicates that the effects of the infection may not end when the fever breaks. In the study, 32 percent of people diagnosed with bipolar disorder had been to the hospital for the treatment of an infection, and five percent had experienced an autoimmune disease related to their infection prior to their diagnosis with bipolar disorder.

The group hopes to identify whether this strong correlation is just a coincidence of hospitalization or if the infection's changes to the body actually make one more susceptible to mood instabilities. Michael Eriksen Benrós, M.D. and leader of this study at Aarhus University, has said, "We can see that the brain is affected, whichever type of infection or autoimmune disease it is. Therefore, it is naturally important that more research is conducted into the mechanisms which lie behind the connection between the immune system and mood disorders."

Source: Benros ME, Waltoft BL, Nordentoft M, et al. Autoimmune Diseases and Severe Infections as Risk Factors for Mood Disorders: A Nationwide Study. JAMA. 2013.