criteria: The virus is able to enter the cell (called a susceptible state) There is a sufficient number of viruses within the cell The virus is able to replicate within the cell (called a permissive state) Hence susceptibility only refers to the fact that the virus is able to get into the cell, via having the proper receptor(s), and as a result, despite the fact that a host may be susceptible, the virus may still not be able to cause any pathologies within the host. Reasons for this are varied and may include suppression by the host immune system, or abortive measures taken by intrinsic cell defenses. See also Mathematical modelling in epidemiology Immunity (medical) Immune system Immunization Bugchasing and giftgiving References Epidemiology