I think it is very painful, but actually doctors can miss the diagnosis.
My older child was hospitalized for a severe infection and kept telling the doctors she thought she had thrush as she'd had it before. It was so painful she was having trouble speaking and they ordered a speech therapist evaluation (!). I told the therapist I thought she just had thrush and she agreed, and still the doctors wouldn't give her the medication.
Anyway, DD ended up in the ER two days after she was released. It turned out that the antibiotics they'd given her had stopped being effective. The ER doctor is my heroine--she went over the labs, said she needed a different antibiotic, and took one look inside her mouth and pronounced that she had thrush.
By the way, this was a good teaching hospital, but it was the first week of July, a time in which you are warned to avoid all teaching hospitals as all the residents change.