Steed and Keel investigate when tests of an experimental vaccine result in a deadly disease. The two agents learn that a scientist, jealous of the inventor of the vaccine, is responsible for the biological disaster. Believing that Steed has been infected by it, Keel races against time to find an antidote -- but the "real" climax occurs after Steed has been given a clean bill of health. Written by Lester Powell, "The Deadly Air" was originally broadcast on December 16, 1961. ~ Hal Erickson, Rovi