Candida - what if there is too much it? 

http://www.thefunctionalgutclinic.com/all-about-candida/
The overgrowth of Candida is called candidiasis, which typically manifests in the immunocompromised, such as patients undergoing chemotherapy, Type 1 diabetics and those with HIV. Also, it can present after the use of steroids and antibiotics – yeast compete with intestinal bacteria for space – a course of antibiotics may kill off bacterial flora allowing the Candida to move in like squatters. Excess Candida stimulates mast cells to release histamine causing irritable IBS-like symptoms. There is doubt as to whether overgrowth can lead to ‘leaky gut syndrome’ but it is thought Candida could damage the intestinal barrier causing inflammation.