Hello Fastenal... I'm no medical scientific whizz, but when a virus
enters the body, its sole purpose for doing so is to take over the
protein factory of the cell to make copies of itself. The infected cell
ultimately dies, spewing it's abundance of newly made viruses into the
blood-stream. The body counteracts this foreign invasion by causing the
body's B-cells (The one's specifically suited to produce the appropriate
antibody to the virus) to multiply. These mass produced B-cells produce
the antibodies which are able to bind with the viruses and thus either
destroy them, or render them incapable of entering and thus infecting
other cells. During this process other blood cells are alerted to the
presence of the invader and basically eat (phagocytosis), and mop up the
inert viral material. ...Some viruses are able to escape by lying
dormant in cells, or incorporating their genetic material directly into
the host's cell DNA. ...However, many are initially released out of the
body, and ultimately go on to infect others. ...The body usually can
create an immunity to viral infection, as B-memory cells are also
produced during the immune response. These cells lie dormant in the
body, but are able to react "more" quickly should the same foreign
invader ever re-gain access. ...Such is the case, for example, when one
is (or used to be) vaccinated for small-pox. ...The memory cells do
reduce, or become less effective over time, thus a "booster" shot is
required periodically to stimulate their production.    A virus is host
specific. In other words, the cell must have appropriate receptors
sticking out by which it can attach itself (in order to inject its
genetic material). This is why humans are immune to many diseases that
animals get, and vice-versa -- even though they may be exposed to the
virus.  ..I believe there are other factors released during the
infection. "Interferon", for example, which helps prevent the virus from
infecting other cells.
..All this is off the top of my head from 1st year biology (taken a few
decades ago). I would suggest a google search to gain a greater
understanding of the nature of viruses and infection.    ...Jon