We assume that a low similarity in receptor sequences disfavors infection despite the existence of a common receptor. For example, enterovirus infects only Sus scrofa (pig); it does not infect Rattus norvegicus (rat) or Homo sapiens (human) because of the high transmission barrier. Similarly, for leukovirus, only Gallus gallus (chicken) is infected as a primary reservoir; because of the high transmission barrier, R. norvegicus and H. sapiens are not infected. These results imply that for non-infection cases, species barriers exist, and the propensity to cross the barrier is determined by the sequence similarity between the potential and primary host receptors.