“…First, there is considerable mathematical interest in models of recurrent epidemic behaviour as a consequence of their non-linear dynamical properties (see Dietz, 1976;Nussbaum, 1977;Busenberg & Cooke, 1978;Green, 1978;Smith, 1978;Yorke et al 1979;Grossman, 1980;Gripenberg, 1980;Stech & Williams, 1981; Hethcote, Stech & van den Driessche, 1981Schwartz & Smith, 1983;Aron & Schwartz, 1984). The mass-action non-linearity, combined with incubation delays in the course of infection, may induce simple cycles or two-point and higher-order cycles (moving into chaos), depending on parameter values and precise model structure.…”