The rodent hosts of Arenavirus species are chronically infected with virus; however, the viruses do not appear to cause obvious illness in them. Some Arenavirus types appear to be passed from mother rodents to their offspring during pregnancy, and thus remain in the rodent population generation after generation. Most infections are transmitted among adult rodents, likely via fighting and inflicting bites. Only a portion of the rodents in each host species is infected at any one time, and in many cases only in a limited portion of the host's geographical range. Viruses are shed into the environment in the urine or droppings of the infected hosts.