Several assumptions were made in the formulation of these equations: First, an individual in the population must be considered as having an equal probability as every other individual of contracting the disease with a rate of formula_82 and an equal fraction formula_83 of people that an individual makes contact with per unit time. Then, let formula_75 be the multiplication of formula_82 and formula_83. This is the transmission probability times the contact rate. Besides, an infected individual makes contact with formula_83 persons per unit time whereas only a fraction, formula_88 of them are susceptible.Thus, we have every infective can infect formula_89 susceptible persons, and therefore, the whole number of susceptibles infected by infectives per unit time is formula_90. For the second and third equations, consider the population leaving the susceptible class as equal to the number entering the infected class. However, a number equal to the fraction formula_76 (which represents the mean recovery/death rate, or formula_92 the mean infective period) of infectives are leaving this class per unit time to enter the removed class. These processes which occur simultaneously are referred to as the Law of Mass Action, a widely accepted idea that the rate of contact between two groups in a population is proportional to the size of each of the groups concerned. Finally, it is assumed that the rate of infection and recovery is much faster than the time scale of births and deaths and therefore, these factors are ignored in this model.