In this chapter, we begin to consider ecoepidemic models. These are demographic models accounting for interactions among diﬀerent populations of the types described in the preceding Chapter 4 in which a disease also spreads. To our knowledge, the ﬁrst papers that appeared in this ﬁeld are by Hadeler and Freedman (1989), Venturino (1992, 1994), and Beltrami and Carroll (1994). The name of the ﬁeld is due to Joydev Chattopadhyay, following an earlier suggestion of the late Ovide Arino. Since those ﬁrst investigations a number of papers have appeared in the literature. Among the more recent contributions to the ﬁeld, we mention also Chattopadhyay and Arino (1999), Chattopadhyay et al. (2002), and Arino et al. (2004). In particular these ecoepidemiological models turned useful for the analysis of harmful algal blooms, see Chattopadhyay and Pal (2002) and Chattopadhyay et al. (2002; 2003; 2004). Ecoepidemic models involving more complex phenomena among the populations involved have been investigated with more sophisticated mathematical techniques in Venturino (2001; 2002; 2004; 2006; 2007), Haque and Venturino (2006a; 2006b; 2007), and Keller and Venturino (2007).