A doctor is the best source to address this question. He or she can order the necessary testing to better determine if something sinister is taking place. Nonetheless, from a nutritional perspective, a couple of thoughts come to mind.
One, many people are deficient in certain vitamins: vitamin D is one vitamin that particularly comes to mind. Therefore, consider a whole food vitamin supplement, one that includes at least 1,000 IU of vitamin D.
Two, consider taking a probiotic. Probiotics introduce additional amounts of good bacteria to the gut. From what I have read, this can help the immune system better fight off illness. Anyway, I hope this helps.