These notes correspond to a set of lectures given at the Univer- sity of Alberta during the spring semester, 1973. The first four sec- tions present a systematic development of a deterministic, threshold model for the spraad of an infection. Section 5 presents some compu- tational results and attempts to tie the model with other mathematics. In each of the last three sections a separate, specialized topic is presented. The author wishes to thank Professor F. Hoppensteadt for making available preprints of two of his papers and for reading and comment- ing on a preliminary version of these...