The cause of yeast infection is an overgrowth of Candida albicans or occasionally Candida glabrata.
There are other yeasts in this family but rarely are C tropicalis, C parapsilosis, C krusei or one of the other Candida species the cause of yeast infection. Candida albicans is a fungi (yeasts are fungi) and it normally lives in our gastrointestinal tract, our mouth, and vagina in harmony with the bacteria that also live in those places.
The presence of the bacteria competing for both food and space keeps the Candida albicans in check. So - Candida is supposed to be present but just be a bit player in the grand scheme of things, not the star!
Yeast infection causes an awful lot of unhappiness - the Mayo Clinic estimates that 3 out of every 4 women will have an infection in their lifetime. Here's a link to check for yeast infection symptoms.
A number of things, whilst not actually causing the infection, definitely increase a woman's chances of developing this complaint.
Here's 10 lifestyle choices that can trigger an outbreak.
Yeast infection is just one of the possible causes for vaginitis so that if you have what you believe is a yeast infection but it keeps coming back, you really should look at some of the other possible causes that produce symptoms similar to yeast infection.
National Institute of Allergy and Infectious Diseases