It frequently occurs as a problem secondary to other infections, but also can develop on its own - from eating food that has been left out too long. Stress also plays a role in suppressing the immune system of birds. Antibiotic treatment can also wipe out the gut flora, good and bad and can be a contributing factor to the development of yeast infections.
Sick bird syndrome develops slowly, where the bird becomes less and less active, sleeps more, fluffs up, and loses interest in its environment. It may attack the throat respiratory tracts, reproductive organs, and vent. Rattling may also occur. Candida can escalate into secondary infections.
Keep the environment clean; remove fresh food before it gets old. Administer a good pre-biotic/probiotic on a regular basis. INTESTUM is frequently suggested during and after antibiotic treatment to prevent yeast infections. It will regenerate gut flora, thus creating competition with the yeast for resources and "starving" it out. The flora so produced will also make the gut more acidic and thus less hospitable for yeasts
Avoid routine treatment of your flock with antibiotics as not only will this increase drug-resistance and affect the natural immunity of your birds, but it can also lead to yeast infections.