The honor of presenting the 25th Charles Franklin Craig Lecture is enhanced for me in two ways. For one, my subject, on account of Col. Craig's own concern with the immunological aspects of parasitic infections, would, I like to think, have had an unusual interest for him in depicting one of the directions in which wormhost relationships are now being so actively studied. For the other, it allows the list of topics of lectures given from the Craig rostrum to include another dealing with the helminths. The most recent was the 18th at Louisville in 1953 by Dr. H. E. Meleney on “Problems in the Control of Schistosomiasis.” Before that in the 9th at St. Louis in 1944, Dr. L. T. Coggeshall devoted half his lecture to “Filariasis in the Returning Service Man.” The 4th Craig Lecture in Memphis in 1939 by Dr. W. H. Taliaferro was on “The Mechanism of Acquired Immunity to Metazoan Parasites.”