Some of you may remember that, four years ago, I inflicted upon you a paper dealing with almost the same question. It was entitled “Some Examples of Neurotoxic Bacterial Action.” Since that time I have continued, in the Laboratory of the Scottish asylums, the practical study of bacterial infections on a fairly wide basis, and I believe it is now possible to define with something approaching exactness the part which such infections play in the causation of mental disorders, including insanity. It can now be shown that this part is a very much larger one than at present is generally believed. The same can, however, be said with equal truth regarding the relation of infections to common maladies.