A status-based approach to multiple strain dynamics.

We present and investigate a new model for cross-immunity. Past models classify hosts according to their infection history. Here we represent hosts through their status: their current ability to respond to strains. This framework allows a different, a wider, and a more biologically interpretable range of forms of cross-immunity to be studied. Using this new form of cross-immunity we then consider a previously studied case of four strains, each of which confers partial immunity to two of the others. In this interesting special case, with applications to the genetic maintenance of strain diversity, we can make substantial analytical progress. We present methods for exploiting the symmetries of the system to show that only a particular invariant subspace need be considered for characterizing the dynamics of the whole system. A complete bifurcation structure is given for this subspace. In contrast to systems previously studied, this system does not exhibit sustained oscillations for any set of parameter values.