Advanced Studies in Biology, Vol. 3, 2011, no. 2, 55 - 61 A Mathematical Model for an Epidemic without Immunity Mahboobeh Mohamadhasani and Masoud Haveshki Department of Mathematics, Hormozgan University P.O. Box 3995, Bandarabbas, Iran ma.mohamadhasani@gmail.com ma.haveshki@gmail.com Abstract In this paper we introduce a model for some diseases which have temporary immunity. It means after recovery, there is immunity but it is not permanent. In this disease, the people are divided into some groups, susceptible, infective, immune and dead people. It is to be noted that we pay attention to people who are born or die because of any reasons except of the disease too. Then we get the equilibrium point and prove it is unstable. Mathematics Subject Classification: 37N25 Keywords: Susceptible and infective people, SIRS Model, equilibrium point, stability 1 Introduction The diseases and any studying about them can be very helpful and important for human beings. Specially, mathematical modeling can be a good and suit- able instrument for having a better life. The basic SIR model has a long history. At first Kermak and Mckendrich introduced SIR model in 1927 [6]. Now, SIR model is developing more and more that you can even find it discussed in some introductory calculus text books [5]. SIR model can be very useful and helpful in characterizing some disease. More numbers of these mathematical models can be seen in [2,3,7,8,9]. We concentrate on some disease which are very fa- mous because of their properties meaning temporary immunity. It means every body who get the disease, after the recovery, get immunity for a short time, not for ever and ever. Also, it is notable that the disease is contagious where the disease is transmitted from the infective people to susceptible people. Also