Incredibly minute particles of matter called viruses are responsible for more than half the world's diseases. Never even seen until the invention of the electron miscroscope, viruses now are the object of intensive scientific scrutiny. World leader in virus research is Toronto's Princess Margaret Hospital. This program studies the work of the hospital's team, under the direction of Dr. L. Siminovitch. Special guest is Dr. K. Rozee of Connaught Laboratories.