Ticks and discease : susceptible hosts, reservoir hosts, and vectors

Tick species vary in the number of hosts fed upon during the life cycle, the specificity they have for certain host taxa, and the mode of transmission, among other life history details. What, then, is the definition of a 'natural focus' of a tick disease in an endemic region? It is the smallest possible intersection of a habitat (or microhabitat) in which the whole cycle of a parasitosis may be achieved in the wild, including the reservoir hosts, nutritional hosts, ticks and transmissible pathogens. Humans or domestic animals may reveal such a focus, when entering one. They become sick, and one observes the acquired disease clinically or serologically. Such a natural, focus may 'export' infected ticks with hosts. If such ticks are infected and leave the hosts out of a natural focus, they can begin new foci. I call such emigrations a 'pulsation' from an original focus. Young rodents, birds, or migrating large mammals may play an important role in originating new foci of diseases and then creating new endemic regions. The possibility that ticks transmit their pathogens through the eggs, over many generations (transovarial transmission), means that ticks act not only as vectors but also as reservoirs of the pathogens they transmit.