When is a person diagnosed with AIDS?
AIDS is diagnosed by measuring how many CD4 cells are found in a cubic millimeter of blood.

A normal adult has between 400 and 1600 CD4 cells per cubic millimeter of blood.

A person in the nonsymptomatic stage of HIV has between 500 and 600 cells per cubic
millimeter of blood.

Every time the CD4 count drops by 100, a person's relative risk of acquiring AIDS
doubles.
When an HIV-positive person has both a low CD4 count and what doctors call an "AIDSdefining illness" or "opportunistic infection," that person has AIDS. Early symptoms of
worsening HIV infection include night sweats, fatigue, weight loss, and hugely swollen lymph
nodes. Some of the most common life-threatening, AIDS-defining diseases are:

Kaposi's sarcoma (KS) -- a skin cancer characterized by purple lesions. Before AIDS,
KS was usually seen in older men of Eastern European descent. This once highly visible
sign of the illness is less common now than it was during the '80s and early '90s because
doctors have learned to prevent or control it.

Pneumocystis carinii pneumonia (PCP)-- a rare form of pneumonia seen in people with
suppressed immune systems. Symptoms include extreme shortness of breath; treatment
requires heavy doses of antibiotics combined with oxygen therapy. PCP is currently a
common infection in people with AIDS, and many suffer several bouts of it before dying.

Candidiasis, or thrush -- an explosive fungus growth that can occur in the mouth, throat,
windpipe or on the skin. It appears most frequently in men as a white coating of the
mouth and causes burning, a bad taste and lack of appetite. Women can also get it in the
mouth, but often one of the first warning signs for women is a chronic vaginal candidiasis
that does not respond to treatment. The fungus naturally lives in the body in small
quantities, but without a healthy immune system to keep it in check, it can run rampant.
In rare cases, candidiasis spreads throughout the body and becomes a life-threatening
condition.

Toxoplasmosis -- a parasite that infects the brain and can cause brain damage and
blindness. In the United States alone, up to 60 million people are thought to be infected
with the parasite, but the immune system normally keeps it under control. Before AIDS,
toxoplasmosis was considered a minor danger for pregnant women.

Cytomegalovirus (CMV) -- a member of the herpes family of viruses and one of the
leading causes of blindness and death in people with AIDS. It is another virus that is
common in adults, but is normally held in check by a healthy immune system. CMV can
infect any part of the body, but in people with AIDS, it generally attacks the eye.
Doctors can treat each of these conditions and most other opportunistic infections, but when HIV
has destroyed the immune system, these diseases can take hold with fatal results. In the late
stages of AIDS, HIV can begin attacking other cells besides the immune system's. Scientists
don't entirely understand HIV's impact on the nervous system, but frequently, individuals with
AIDS begin to experience dementia, problems with balance, and an inability to move their legs
and arms. HIV can also attack a person's internal organs
Almost immediately upon infection, the immune system recognizes that it is under attack. Many
people experience flu-like symptoms, sometimes with a rash, two to three weeks after being
infected. This is the body reacting to the invading HIV and mounting a terrific immune response,
producing up to a billion helper and killer T-cells a day. At this point, there is enough virus in the
blood that HIV can be diagnosed through a blood test.
Because the virus infects CD4 cells, stimulating the immune system means activating infected
CD4 cells, which then produce more viruses. Those viruses in turn go out and infect more of the
newly created CD4 cells.
Because they have such large armies, the body and HIV can battle each other for up to 10 years
or longer. During this time, the HIV-infected person will feel totally healthy. But since the virus
is incredibly active, the person can spread HIV to anyone with whom he or she exchanges bodily
fluids, such as blood, vaginal fluid, semen or breast milk.
Eventually HIV gains the upper hand and begins to kill off more CD4 cells than the body can
produce. Without the CD4 cells, the B-cells and killer T-cells lack instruction about where to go
and what to look for, and the body's immune response becomes less and less effective
How do you get HIV?
Unlike a flu virus, which can live for hours outside a human body and can be easily spread
through the air or other surfaces, HIV can't live outside a body for more than a few
minutes, nor can it be killed by soap nor is it easily transmissible. It can only be passed
through blood or through contact between mucosal surfaces. The only bodily fluids that
contain HIV in significant amounts are semen, vaginal fluid, and breast milk. HIV cannot
be spread through contact with saliva, tears or sweat.
The primary ways HIV spreads are:

Sex (either vaginal or anal)

Sharing of contaminated needles

From mother to child at birth

Breastfeeding
In rare cases, HIV can also be spread through oral sex, especially where the throat is already
inflamed.
After HIV enters the body, it hunts down and penetrates the CD4 cell, which functions as the
immune system's quarterback: It tells the other cells what they need to do to rid the body of an
invader. HIV hijacks the CD4 cell and uses the cell's machinery to make new virions, or virus
particles. Here's how:
HIV disguises itself behind a layer of sugars, which makes it appear harmless (although some
cells do recognize and destroy HIV particles). Because immune system cells are constantly
circulating in and out of the lymph organs, the HIV-infected cell will soon carry the virus
straight to the stronghold of the lymphatic system, where CD4 cells are most abundant. Once
there, HIV begins replicating and infecting CD4 cells very quickly. It replicates itself billions of
times each day.
Imagine HIV as a sphere with spiky proteins all over its surface. In order to infect a CD4 cell,
one of those proteins, known as gp120, fits together with a receptor protein, CCR4, on the CD4
cell's surface, like a key fits into a lock. A second protein, called gp41, then locks onto a second
receptor protein, CCR5. Once both connections are made, the virus can penetrate the outer wall
of the cell and insert its viral proteins and RNA.
After penetrating a CD4 cell, the virus goes to work turning it into an HIV factory. An
enzyme called reverse transcriptase transcribes the viral RNA into DNA. Then another enzyme,
known as integrase, cuts apart the cell's own DNA, and inserts the viral DNA. The virus has
become a provirus -- a viral cell that can lie dormant for years until it's activated. Once the cell is
activated, its DNA tells the cell to produce more copies of the RNA and other proteins needed to
make new HIV virions. After the copies are made, they gather at the cell's surface and form buds
before finally disconnecting from the host cell. Once they're free, an enzyme called protease cuts
the new proteins and RNA into small, usable pieces. The new virions are now mature, and ready
to do battle with the body.