SIR Epidemic Model: Anti-Infectious

<p>We revisited the classical SIR epidemic model. For nonnegative initial conditions, the solutions are bounded below by zero and above by the total population, and hence the solution exists for any positive time, and the system has a global solution. It may be taken as evident that it has also a global solution for arbitrary initial conditions. That is true, but for a particular case where an initial condition of infectious is negative, the solution blows up in finite time, and therefore the system does not possess a global solution. We speculate with the biological or epidemiological meaning of a negative number of infectious at an initial time.</p>