What Causes Recurring Sickness, Headaches And Sore Throat?

My daughter has been sick multiple times this school year causing her to miss a lot of school. Illness is always upper respiratory. Lab work only showed low vitamin D level. All other lab work normal. Lyme test negative but was .46. Shouldn t that be 0 if there is no evidence of lyme? Other symptoms include headaches and sore throat. Any ideas?