Fig. 1.

Schematic diagram illustrating the MSMH model. Susceptibles are denoted by S, infectious classes by I, and super-strain infectious classes by J. Each species’ classes and parameters for are subscripted by b (birds), h (humans), and p (pigs). All host species have a SIS structure, where recruitment is logistic of the entire population into the susceptible subclass and transfer from infected subclasses by recovery into the susceptible subclass or disease-induced death. The birds have a basic structure, humans have a super-infection structure, and pigs have a coinfection structure with recombination. Solid arrows represent the transfer due to infection within a single host population. Dashed arrows represent the direction of interspecies infectivity from human-to-pig with the endemic human strain, bird-to-pig with the endemic avian strain, and pig-to-human with the super-strain. A description of the variables and parameters is given in Sections 1 and 3 of the Appendix.