Why is my child getting sick so often?

Before I go further, just for the record, yes, I have taken her to the doctor and yes, we are scheduled to see an E.N.T. She takes multi vitamins and fish oil gummies on her doctor's recommendation.

My daughter is not quite 2 1/2 years old. In June, she started getting sick. She broke out in hives and had a fever and during this illness, had a seizure due to her fever spiking (I know that's not particularly dangerous. Just trying to describe the symptoms when this all started). The doctors couldn't figure out what caused it but her tonsils swelled quite a lot. She was tested for strep,and the flu and they did a V.C.U.G. test, (catheter used to fill her bladder with dye solution to be sure her urine wasn't backing up into her kidneys). CAT scans were done, chest ex-rays, and blood tests. Nothing conclusive was found, but after 2 antibiotics and antihistamine for the hives, it seemed to get better. It wasn't a week later when she got sick again. Upper respiratory infection. More antibiotics. I'm not so sure that ever really went away before she got another throat infection, high fevers etc… This continued and her tonsils never shrunk but from what I understand, when a child gets ill sometimes the tonsils can remain swollen for years or even for the rest of that person's life. They're still big and red but most of the time, it doesn't seem to bother her, and she's never had an ear infection in her life. She does however, snore and tends to salivate excessively, like it's almost too big a bother to swallow. She has no mental impairment and her speech is above average.

Anyway, she's been sick since June with only two days that I can remember when she felt really well. She has a perpetually runny nose and a cough that never goes away. A month ago it was really bad again. Her tonsils were so inflamed that they looked like someone poured boiling oil down her throat. They looked like they had blistered and the blisters broke, so there was dead skin hanging from them. She was very lethargic and wouldn't eat or drink, and her fevers were hitting 104-105. We had her in the E.R. twice and again, they did all of the tests (catheter included but no V.C.U.G.) and found nothing. I took her to her regular doctor and mentioned that if I didn't know better, I'd think she had Mono. Sure enough, she had an 'Epstein Barr-type virus'. Three antibiotics later, (for the secondary respiratory infection) she was better (not well, but better than that). Two weeks ago, she had a stomach virus that was going around the day care and was vomiting. That only lasted a day. Last week she caught another stomach virus that was going around her day care (this time it was diarrhea) and most kids were over it in two days. Hers lasted five. She finally went back to the day care today for the first time in a week, and tonight she has a fever of 104. The kid is just never well! I requested that we be referred to an E.N.T. and we're scheduled for that on the 6th, but if it's not her tonsils causing this, does anyone have a guess what might be? I am looking for something I may just be missing that I might bring up to the doctor. Yes I know…the doctor is the expert, but sometimes when you don't get real answers, you really have to look deeper yourself. Doctors are only human and see hundreds of patients a year, so just like with the Mono diagnosis, they can miss things too. Any ideas?