This (subscription required, abstract) is a very interesting study, published in the current issue of PNAS (Proceedings of the National Academy of Sciences). The short version? An international team of researchers headed by University of Wisconsin-Madison's Marulasiddappa Suresh took a major step forward in understanding how the immune system remembers events.
If you've ever wondered why people who've had chicken pox, the measles or a host of other illnesses rarely find themselves visiting the doctor with the same complaint, strap yourself in and read on.