Yeast infections, or candidiasis,  is a fungal infection (mycosis) of any of the Candida species, of which Candida albicans is the most common.

Candidiasis encompasses infections that range from superficial, such as oral thrush and vaginitis, to systemic and potentially life-threatening diseases.

Because yeast feed on sugar, limiting sugar in the diet is the first step to reducing occurence. Adding high-fiber foods that promote healthy microflora in the gut (like Fructooligosaccharides) is the second step. Eating anti-fungal foods like garlic, onions, and oregano is also helpful to restore balance and eliminate yeast infections.  

References

Walsh TJ, Dixon DM (1996). "Deep Mycoses", in Baron S et al eds.: Baron's Medical Microbiology (via NCBI Bookshelf), 4th ed., Univ of Texas Medical Branch. ISBN 0-9631172-1-1.