The work of Lotka and Volterra,who arrived independently at several models including those for predator-prey interactions and two- species competition had a profound effect on the field now known as population biology. They were among the first to study the phenomena interacting species by making a number of simplifyin that led to nontrivial but tractablem a problems. since their pioneering work,many other notable contributions were made. Among these is the work of Kermack and McKendrick (1927),who addressed the problem of out breaks of epidemics in apopulation