Tanya, Oral Thrush is a yeast infection(fungus)causes can be from many things,
low immune system-
antibiotic use-
steroids,Asthma medications-
Thrush (Candida)can be found in the mouth , on the tongue,down the throat,and in the Esophagus(wind pipe). The balace of microbes become disturbed and nursing mothers can develope Candida infection on their Breasts (Mastitis). Ask your chemist for Nystatin liquid oral or Nilstat, check dosage for your little one though. Hope your little one is feeling much better very soon,Tanya,good luck.