function rhs=epidemic_rhs(t,y)
% the one-population epidemic right hand sides
% susceptibles S(t) = y(1)
% infectives I(t) = y(2)

global alpha beta gamma



S = y(1);
I = y(2);
R = y(3);

rhs(1) = alpha*S - beta*I*S;
rhs(2) = beta*I*S - gamma*I;
rhs(3) = gamma*I;

rhs = rhs';