  In this paper we propose a method to study a general vector-hosts
mathematical model in order to explain how the changes in biodiversity could
influence the dynamics of vector-borne diseases. We find that under the
assumption of frequency-dependent transmission, i.e. the assumption that the
number of contacts are diluted by the total population of hosts, the presence
of a competent host is a necessary condition for the existence of an endemic
state. In addition, we obtain that in the case of an endemic disease with a
unique competent and resilient host, an increase in its density amplifies the
disease.
