[1] R. M. Anderson, R. M. May: Vaccination against rubella and measles; quantitative investigations of different policies. J.  Hyg. 90 (1983), 259–325.

[2] R. Bellman, K. L.  Cooke: Differential Difference Equations. Academic Press, New York, 1963. MR 0147745

[3] M. C. Boily, R. M.  Anderson: Sexual contact patterns between men and women and the spread of HIV-1 in urban societies in Africa. IMA J. Math. Appl. Med. Biol. 8 (1991), 221–247.

[4] S. N. Busenburg, P. van den Driessche: Analysis of a disease transmission model in a population with varying size. J.  Math. Biol. 29 (1990), 257–270. MR 1047163

[5] S. N. Chow, J. K.  Hale: Methods of Bifurcation Theory. Springer-Verlag, New York, 1982. MR 0660633

[6] R. D. Driver: Ordinary and Delay Differential Equations. Applied Math. Sciences, Springer-Verlag, New York, 1977. MR 0477368 | Zbl 0374.34001

[7] G. Gandolfo: Mathematical Methods and Models in Economic Dynamics. North Holland, London, 1971. Zbl 0227.90010

[8] K. Gopalsamy: Stability and Oscillations in Delay Differential Equations of Population Dynamics. Academic Publisher, Dordrecht-Boston-London, 1992. MR 1163190 | Zbl 0752.34039

[9] D. Greenhalgh, R.  Das: Modelling epidemics with variable contact rates. Theor. Popul. Biol. 47 (1995), 129–179.

[10] J. K. Hale, S. M. V.  Lunel: Introduction to Functional Differential Equations. Applied Math. Sciences  99. Springer-Verlag, New York, 1993. MR 1243878

[11] H. W. Hethcote,J. A.  Yorke: Gonorrhoea transmission dynamics and control. Lecture Notes in Biomathematics, Vol. 56, Springer-Verlag, New York, 1974. MR 0766910

[12] H. W. Hethcote, D. W. Tudor: Integral equation models for endemic infections diseases. J.  Math. Biol. 9 (1980), 37–47. MR 0648844

[13] H. W. Hethcote, H. W. Stech, and P. van den Driessche: Nonlinear oscillations in epidemic models. SIAM J.  Appl. Math. 40 (1981), 1–9. MR 0602496

[14] H. W. Hethcote, H. W. Stech, and P. van den Driessche: Stability analysis for models of diseases without immunity. J.  Math. Biol. 13 (1981), 185–198. MR 0661676

[15] H. W. Hethcote, M.  Lewis, and P. van den Driessche: An epidemiological model with a delay and a non-linear incidence rate. J.  Math. Biol. 27 (1989), 49–64. MR 0984225

[16] J. A. Jacquez, C. P. Simon, J.  Koopman, L. Sattenspiel, and T. Perry: Modelling and analyzing HIV transmissions: The effect of contact patterns. Math. Biosci. 92 (1988), 119–199. MR 0975856

[17] M. Kalecki: A macrodynamic theory of business cycles. Econometrica 3 (1935), 327–344.

[18] W. W. Leontief: Lags and stability of dynamic systems. Econometrica 29 (1961), 659–669. MR 0145984

[19] N. MacDonald: Time Lags in Biological Models. Lecture Notes in Biomathematics Vol. 28. Springer-Verlag, Berlin-Heidelberg-New York, 1979. MR 0521439

[20] R. M. May, R. M.  Anderson, and A. R. McLean: Possible demographic consequences of HIV/AIDS epidemics I.  Assuming HIV infection always lead to AIDS. Math. Biosci. 90 (1988), 475–505. MR 0958153

[21] J. E. Marsden, M.  McCracken: The Hopf Bifurcation and Its Applications. Springer-Verlag, New York, 1976. MR 0494309

[22] A. Nold: Heterogeneity in disease transmission modelling. Math. Biosci. 52 (1980), 227–240. MR 0596176

[23] H. R. Thieme: Global asymptotic stability for epidemic models. Equadiff 82. In: Lecture Notes in Mathematics Vol.  1017, H.  W.  Knobloch, K.  Schmitt (eds.), Springer-Verlag, Berlin, 1983, pp. 608–615. MR 0726617

[24] H. R. Thieme: Local stability in epidemic models for heterogeneous populations. In: Mathematics in Biology and Medicine. Lecture Notes in Biomathematics Vol. 57, V. Capasso, E.  Grosso and S.  L.  Paveri-Fontana (eds.), Springer-Verlag, Berlin, 1985, pp. 185–189. MR 0812889 | Zbl 0584.92020