Candida is a yeast organism, part of a group of similar creatures which includes funguses and moulds. In many people it is present in the bowel as a commensal which means that it lives in harmony with many other bugs in the bowel and causes no harm.
Under certain circumstances it can change to an aggressive form which sets up infection, penetrating the cells in the bowel wall where it roots and becomes difficult to dislodge. This can cause damage to the bowel lining and this interferes with its normal filtering function, allowing inappropriately large or toxic molecules to enter the bloodstream and cause illness. Many experts feel that, as it is impossible to confirm the overgrowth of Candida by direct vision or standard testing, that the syndrome should be called Fungal type dysbiosis as other organisms could theoretically be the cause of the problem.
Factors which suggest Candida include a history of vaginal thrush, steroid use or the use of antibiotics which kill off the competition allowing the yeast organisms to take over the territory.