What causes thrush in a 20 month old?

my sons almost 20 months old and he has thrush the doctor gave him a medicine for it. im just wondering what caused it . just wondering cause i though only little babies get it but of course im wrong. i just feel so bad for my little guy cause i looked in his mouth right now and it looks so pain ful. hes starting to eat more and is not so fussy but when will it look better we went to the doctor yesterday.