function sir =sir(t,y)
a = .01;
b = .1;
sir(1) =-a*y(1)*y(2);
sir(2) = a*y(1)*y(2)-b*y(2);
sir(3) = b*y(2);
sir = [sir(1) sir(2) sir(3)]';

to = 0;
tf =14;
yo = [763 1 0];
[t y] = ode45('sir',[to tf],yo);
plot(t,y(:,1),t,y(:,2),t,y(:,3))
xlabel('time')
ylabel('susceptible, infected, recovered')
legend('susceptible', 'infected', 'recovered')