My ds had a run of while back. He is rarely sick so I was quite concerned. His glands were up continuously for months as well as him getting school sores and a nose infection, sore throat and temp off and on for a few months. The dr checked him out thoroughly and said it was most likely just that his immune system was very low and recommended a high dose of antibiotics and a good multi vitamin. He is 5 too. He is fine now, I think with school being hectic this time of the year he was just run down, he was more tired than usual too. And same thing as you op, no one else was sick in the family.