[1] Al-Ajam, M. R., Bizri, A. R., Mokhbat, J., Weedon, J., Lutwick, L.: 
Mucormycosis in the Eastern Mediterranean: a seasonal disease. Epidemiol. Infect. 134 (2006), 341-346. 
DOI 10.1017/S0950268805004930 
[2] Anderson, R. M., May, R. M.: 
Population biology of infectious diseases, Part 1. Nature 280 (1979), 361. 
DOI 10.1038/280361a0 
[3] Anderson, R. M., May, R. M.: Infectious Diseases of Humans, Dynamics and Control. Oxford University Oxford (1991).
[5] Diekmann, O., Heesterbeek, J. A. P., Metz, J. A. J.: 
On the definition and the computation of the basic reproduction ratio $R_0$ in models for infectious diseases in heterogeneous populations. J. Math. Biol. 28 (1990), 365-382. 
DOI 10.1007/BF00178324 | 
MR 1057044 
[6] Diekmann, O., Heesterbeek, J. A. P.: 
Mathematical Epidemiology of Infectious Diseases: Model Building, Analysis and Interpretation. John Wiley & Sons Chichester (2000). 
MR 1882991 
[7] Driessche, P. van den, Watmough, J.: 
Reproduction numbers and sub-threshold endemic equilibria for compartmental models of disease transmission. Math. Biosci. 180 (2002), 29-48. 
DOI 10.1016/S0025-5564(02)00108-6 | 
MR 1950747 
[8] Earn, D. J. D., Dushoff, J., Levin, S. A.: Ecology and evolution of the flu. Trends in Ecology and Evolution 17 (2002), 334-340.
[10] Gaines, R. E., Mawhin, J. L.: 
Coincidence Degree, and Nonlinear Differential Equations. Springer Berlin (1977). 
MR 0637067 | 
Zbl 0339.47031 
[12] Herzog, G., Redheffer, R.: 
Nonautonomous SEIRS and Thron models for epidemiology and cell biology. Nonlinear Anal., Real World Appl. 5 (2004), 33-44. 
MR 2004085 | 
Zbl 1067.92053 
[17] London, W., Yorke, J. A.: 
Recurrent outbreaks of measles, chickenpox and mumps. 1. Seasonal variation in contact rates. Am. J. Epidemiol. 98 (1973), 453-468. 
DOI 10.1093/oxfordjournals.aje.a121575 
[19] Nuño, M., Feng, Z., Martcheva, M., Carlos, C. C.: 
Dynamics of two-strain influenza with isolation and partial cross-immunity. SIAM J. Appl. Math. 65 (2005), 964-982. 
DOI 10.1137/S003613990343882X | 
MR 2136038 
[21] Teng, Z., Chen, L.: 
Permanence and extinction of periodic predator-prey systems in a patchy environment with delay. Nonlinear Anal., Real World Appl. 4 (2003), 335-364. 
MR 1942689 | 
Zbl 1018.92033 
[24] Zhang, T., Liu, J., Teng, Z.: 
Stability of Hopf bifurcation of a delayed SIRS epidemic model with stage structure. Nonlinear Anal., Real World Appl. 11 (2010), 293-306. 
MR 2570549 | 
Zbl 1195.34130