Williams, Greer. The Plague Killers. Charles Scribners Sons, New York: 1969.There is an aphorism in medical science which says: "You see what you look for; you look for what you know." It can work positively or negatively. If you know something exists but is hidden from view, you may be able to espy clues that thereafter will lead you to it. Thus, discoveries in diagnostic method may be made. On the other hand, if you know something exists in some instances and generalize it to be of significance in all, you may be right--or you may gravely mislead yourself and others.