Abstract: As compared to classical epidemiological models multi-strain models show a rich dynamic behaviour, reaching from new thresholds, namely the reinfection threshold in models of partial immunity, for example in influenza, to new deterministically chaotic dynamics, in dengue fever models, and can show evolution to critical thresholds including its large fluctuations for accidental pathogens, e.g. in Neisseria menigitidis epidemiology.

References
[1] Stollenwerk, N., & Jansen, V.A.A. (2003) Meningitis, pathogenicity near criticality: the epidemiology of meningococcal disease as a model for accidental pathogens. Journal of Theoretical Biology 222, 347--359.
[2] Stollenwerk, N., Maiden, M.C.J., & Jansen, V.A.A. (2004) Diversity in pathogenicity can cause outbreaks of menigococcal disease, Proc. Natl. Acad. Sci. USA 101, 10229-10234.
[3] Stollenwerk, N., & Jansen, V.A.A. (2007) Criticality in epidemiology, in Complex Population Dynamics: Nonlinear Modeling in Ecology, Epidemiology and Genetics, eds. Bernd Blasius, Lewi Stone, Jürgen Kurths, (World Scientific Lecture Notes in Complex Systems)
[4] Stollenwerk, N., Martins, J., & Pinto, A. (2007) The phase transition lines in pair approximation for the basic reinfection model SIRI,
[5] Aguiar, M., & Stollenwerk, N. (2007) A new chaotic attractor in a basic multi-strain epidemiological model with temporary cross-immunity, available as: arXiv:nlin/0704.3174.
[6] Massad, E., Chen, M., Ma, S., Struchiner, C.J., Stollenwerk, N., & Aguiar, M. (2008) Scale-free Network for a Dengue Epidemic, Applied Mathematics and Computation 159, 376--381.
[7] Aguiar, M., Stollenwerk, N., & Kooi, B.(2008) Torus bifurcations, isolas and chaotic attractors in a simple dengue model with ADE and temporary cross immunity, Proceedings of 8th Conference on Computational and Mathematical Methods in Science and Engineering, CMMSE 2008, ISBN 978-84-612-1982-7, also available as: arXiv:0806.4576.