Brain fever:

Brain fever afflicts several characters in the Canon, most notably Mrs. Barclay in The Crooked Man and Percy Phelps in The Naval Treaty. What is it? According to a New York Times article and a wikipedia entry, it’s a catchall diagnosis that only vaguely refers to the a swelling of the brain or the very logical fear of a high fever. Today, we’d be more likely to call it hysteria (despite it’s original meaning of an excitement of the uterus) or an anxiety attack, but in Victorian times, it would not be uncommon for a physician to fear his patient’s death during an attack of brain fever.