Birds treated with antibiotics can develop a secondary respiratory infection caused by yeast. Yeast are found everywhere in the environment. It can develop on spoiled food. For this reason, fresh fruits, vegetables and table food should never be left for more than a couple of hours in the bird's cage. In the summer, food can spoil in less than an hour.
Poor hygiene can also spread yeast infections.
Not sterilizing hand feeding equipment, improper cage cleaning and even not washing our hands can spread yeast and bacteria to our birds. Because yeast is in our environment, most healthy birds will not be affected by it. Older birds, very young birds, sick birds and stressed birds can develop yeast infections as secondary infections. These secondary infections put increased stress on their already over-taxed immune system and interfere with the healing process. Birds that have vitamin deficiencies often will develop yeast infections.