This bacteria aids digestion, extracting vitamins from the food we eat. Without it we would die. Candida also exists in the colon. The amount of Candida in a healthy person is quite small. The amount of bacteria is normally quite large.
Candida has no known benefits, but in small amounts it is harmless.
Many of the books on the subject list antibiotics as the primary cause of Candida. While this is certainly a significant cause, many people including many MDs believe that the real cause is immune deficiency. It is still not known why such immune deficiencies occur in people who otherwise appear healthy. In my case, I believe that aspartame and allergies were undoubtedly among the major causes. Low body temperature caused by Wilson's Syndrome was another.