Candidiasis is caused by the opportunistic yeast Candida albicans. It is commonly found in the environment and may be present in small numbers in a normal bird's digestive tract. It may, however, cause illness under certain conditions. Very young birds, especially those on antibiotics, may develop primary candidiasis due to their immature immune systems. Adult birds on long term antibiotics or suffering from malnutrition or other illnesses may develop secondary candidiasis.
Antibiotics change the normal digestive tract flora and allow the organism to overgrow.
Birds affected with beak lesions as a result of Beak and Feather virus often develop secondary candidiasis due to a poor immune system. Candidiasis has also been found to affect the skin, respiratory tract, central nervous system and all other organs. The severity of a candida infection often depends on the age of the bird and the state of its immune system. A very young or very ill bird may initially have a localized candida infection that spreads systemically to the blood, bone marrow, and other organs. Severe stress or immunosuppression can enable a localized infection to become an overwhelming systemic infection.