  In epidemiological modelings, the spectral radius of the next generation
matrix evaluated at the trivial equilibrium was considered as the basic
reproduction number. Also, the global stability of the trivial equilibrium
point was determined by the left eigenvector associated to that next generation
matrix. More recently, the fraction of susceptible individuals was also
obtained from the next generation matrix. By revisiting drug sensitive and
resistant tuberculosis model, the gross reproduction number and the fraction of
susceptible individuals are calculated. Hence, the next generation matrices
shed light to the evolution of the dynamics: the beginning of the epidemics via
the reproduction number and the approaching to the epidemics level via the
asymptotic fraction of susceptible individuals.
