“…As we know, since Bartlett [1] in 1956 predicted a wave of infection moving out from the initial source of infection with a constant speed, epidemic waves have been extensively studied, see for example [10,29,22,45,15,38,40,42,20,12,19,11,23,46]. See also the monograph [30], the survey papers [32,33] and the references therein for more details.…”