Oral thrush and other candida infections can occur when your immune system is weakened by disease or drugs such as prednisone, or when antibiotics disturb the natural balance of microorganisms in your body.

Normally, your immune system works to repel harmful invading organisms, such as viruses, bacteria and fungi, while maintaining a balance between "good" and "bad" microbes that normally inhabit your body. But sometimes these protective mechanisms fail, which can allow an oral thrush infection to take hold.

These illnesses may make you more susceptible to oral thrush infection:

   * HIV/AIDS. The human immunodeficiency virus (HIV) — the virus that causes AIDS — damages or destroys cells of your immune system, making you more susceptible to opportunistic infections that your body would normally resist. Repeated bouts of oral thrush may be the first sign of an HIV infection.
   * Cancer. If you're dealing with cancer, your immune system is likely to be weakened both from the disease and from treatments such as chemotherapy and radiation, increasing your risk of candida infections such as oral thrush.
   * Diabetes mellitus. If you don't know you have diabetes or the disease isn't well controlled, your saliva may contain large amounts of sugar, which encourages the growth of candida.
   * Vaginal yeast infections. Vaginal yeast infections are caused by the same fungus that causes oral thrush. Although a yeast infection isn't dangerous, if you're pregnant you can pass the fungus to your baby during delivery. As a result, your newborn may develop oral thrush.