IN SCIENCE, as in many other walks of life, what is unexpected is often what is most interesting and important. The idea, first mooted in 2011 by Julian Gold of the Prince of Wales Hospital in Sydney, Australia, that HIV infection—or maybe the drug treatment used to fight it—might protect against multiple sclerosis (MS), was certainly unexpected. Now, in a study just published in the Journal of Neurology, Neurosurgery and Psychiatry, Dr Gold has confirmed his suspicion. That is interesting. It may also be important. Read on.