It is believed that psychological stress may trigger IBS in predisposed individuals. Given the high levels of anxiety experienced by people with IBS and the overlap with conditions such as fibromyalgia and chronic fatigue syndrome, a potential explanation for IBS involves a disruption of the stress system. The stress response in the body involves the hypothalamic–pituitary–adrenal axis (HPA) and the sympathetic nervous system, both of which have been shown to operate abnormally in people with IBS. Psychiatric illness or anxiety precedes IBS symptoms in two-thirds of people with IBS, and psychological traits predispose previously healthy people to developing IBS after gastroenteritis. Approximately 10 percent of IBS cases are triggered by an acute gastroenteritis infection. The CdtB toxin is produced by bacteria causing gastroenteritis and the host may develop an autoimmunity when host antibodies to CdtB cross-react with vinculin. Genetic defects relating to the innate immune system and epithelial barrier as well as high stress and anxiety levels appear to increase the risk of developing post-infectious IBS. Post-infectious IBS usually manifests itself as the diarrhea-predominant subtype. Evidence has demonstrated that the release of high levels of proinflammatory cytokines during acute enteric infection causes increased gut permeability leading to translocation of the commensal bacteria across the epithelial barrier; this in turn can result in significant damage to local tissues, which can develop into chronic gut abnormalities in sensitive individuals. However, increased gut permeability is strongly associated with IBS regardless of whether IBS was initiated by an infection or not. A link between small intestinal bacterial overgrowth and tropical sprue has been proposed to be involved as a cause of post-infectious IBS. Small intestinal bacterial overgrowth (SIBO) occurs with greater frequency in people who have been diagnosed with IBS compared to healthy controls. SIBO is most common in diarrhea-predominate IBS but also occurs in constipation-predominant IBS more frequently than healthy controls. Symptoms of SIBO include bloating, abdominal pain, diarrhea or constipation among others. IBS may be the result of the immune system interacting abnormally with gut microbiota resulting in an abnormal cytokine signalling profile. Certain bacteria are found in lower or higher abundance when compared with healthy individuals. Generally Bacteroidetes, Firmicutes, and Proteobacteria are increased and Actinobacteria, Bifidobacteria, and Lactobacillus are decreased.