Re: Why is the HIV virus so frail outside of the body and so deadly in it?

The Human Immunodeficiency Virus (HIV) is an enveloped RNA
virus called a retrovirus.  It has a difficult time surviving
outside the body because its envelope is not a hard,
water-tight shell, it is just a bit of cell membrane.  Its
genome is made of RNA which is much less stable than DNA.

HIV is not really very harmfull to cells compared to most
other viruses.  It does not destroy a cell in a matter of minutes,
as some viruses do.  It is very harmful to the human immune
system over time, and then other viruses and bacteria can
invade the body.  

To use an analogy; Country A might try to take over country B
by invading with a group of men with guns.  The government of
country B sees this war immediately and fights back the invasion
after only a couple of hundred people are killed.  This is like
the flu virus invading a person.  Country C decides to 
ruin the same country B by sending many young men to country
B to join the country B army.  They don't kill anyone, but
they tell the regular soldiers in country B's army to be lazy and
not obey the commands of the army leaders.  After 20 years the
country B army is full of poor soldiers and it gets invaded
by armies from counties E, F and G and cannot fight back.  This
is more like the human immunodeficiency virus invading a person.

Of course viruses can't think.  They don't have a "plan" to
kill anyone.  They are just bits of RNA (or DNA) and protein that
reproduce by invading cells of other organisms.  The immune system
is designed to recognize anything foreign in the body and
destroy it.  Sometimes the immune system does more harm than
good by over-reacting to the problem.  Allergies, asthma, arthritis
and other problems are caused by the immune system taking more
action than is needed.  In the case of HIV and AIDS, the body might
be better off if it ignored HIV.  HIV infects CD4 cells and then
CD8 cells kill the infected CD4 cells more often than the CD4
cells die from the virus itself.  African monkeys of many types
are infected with viruses very similar to HIV, but they don't 
get immune deficiency at all as far as we know.  However if the
virus from one type of monkey is injected into another type
of monkey, they often do get immunodeficiency.  There is a 
complex ecology of virus-host interactions that we do not
yet understand.