AIDS (Acquired ImmunoDeficiency Syndrome) is one of the
worst pandemics the world has ever known. HIV (Human
Immunodeficiency Virus), the virus
that causes AIDS, was first discovered in 1981 in a remote
area of central Africa. It has since swept across the globe,
infecting millions in a relatively short period of time. AIDS
has killed 21.8 million people that we know of, with 3 million
people dying in the year 2000 alone. While many cases go
unreported, the prevalence of the disease is increasing. By
Clearly the AIDS pandemic has had, and will continue to
have, a significant and global impact.
Photo courtesy National Institute of Allergy and
Infectious Diseases HIV, the
virus that causes AIDS, is shown budding out of a human
In this edition of HowStuffWorks,
we will show you how the HIV virus attacks the immune system
and how it causes AIDS. We will also clear up some of the
myths about AIDS and how HIV is transmitted.
Unique Features of HIV The thought of
contracting HIV is frightening. And there is good reason for
that fear -- the disease is presently incurable, it has a high
mortality rate, it spreads quickly and there is no vaccine to
protect against it. In today's world, that combination is
rare. For example, small pox is often fatal, but the
disease has been completely contained through vaccinations.
Tuberculosis is often fatal but can usually be cured
with antibiotics if caught early.
AIDS has been able to infect and kill so many people
because of its unique makeup. Let's look at some of the
features that make this disease so unusual:
HIV spreads by intimate contact with an infected person.
Forms of intimate contact that can transmit AIDS include sexual
activity and any sort of situation that allows blood
from one person to enter another. Especially when you
compare it with the many viruses that spread through the
air, it would seem like the intimacy involved in the
transmission of AIDS would be a limiting factor. However
A person can carry and transmit the HIV virus for many
years before any symptoms show themselves. A person can be
contagious for a decade or more before any visible signs of
disease become apparent. In a decade, a promiscuous HIV
carrier can potentially infect dozens of people, who each
can infect dozens of people, and so on.
HIV invades the cells of
system and reprograms the cells to become HIV-producing
factories. Slowly, the number of immune cells in the body
dwindles and AIDS develops. Once AIDS manifests, a person is
susceptible to many different infections, because the immune
system has been weakened so much by the HIV it can no longer
fight back effectively. HIV has also shown the ability to
mutate, which makes treating the virus nearly impossible.
The last feature in this list is the one that is truly
unique. HIV invades and destroys the immune system -- the
system that would normally protect the body from a virus. HIV
corrupts and disables the system that should be guarding
How HIV Enters the Body In the United
States, given the current distribution of HIV in the
population, there is better than a one in 1,000 chance of
contracting HIV during an unprotected heterosexual encounter,
according to the Centers
for Disease Control and Prevention (CDC). In some
locations, the chances are even higher. Unprotected sex is the
most common way of transmitting HIV. Your chances for
infection increase with each new partner. Here is a list of
ways in which HIV can be transmitted:
Sharing contaminated intravenous needles
Breastfeeding (mother to baby)
Infected mother to fetus during pregnancy or
Blood transfusions (Rare in countries where blood is
screened for HIV antibodies.)
One of the most
prevalent myths about HIV transmission is that mosquitos
or other bloodsucking insects can infect you. There is
no scientific evidence to support this claim. To see why
mosquitos don't aid in the transmission of HIV, we can
look at the insect's biting behavior.
When mosquitos bite someone, they do not inject its
own blood or the blood of an animal or person it has
bitten into the next person it bites. The mosquito does
inject saliva, which acts as a lubricant so that it can
feed more effectively. Yellow fever and malaria can be
transmitted through the saliva, but HIV does not
reproduce in insects, and therefore doesn't survive in
the mosquito long enough to be transmitted in the
Additionally, mosquitos don't normally travel from
one person to another after ingesting blood. The insects
need time to digest the blood meal before moving on.
There is also a slight chance of transmission through
kissing and biting. However, there have been very few cases of
HIV being transmitted through either method. In fact, the CDC
has investigated only one case in which HIV infection was
attributed to open-mouth kissing.
HIV does not transmit through the air or surface contact
like cold and flu viruses do. HIV is a fragile virus and
doesn't survive well outside the human body. This fragility
makes the possibility of environmental transmission very
remote. Outside of a host cell, HIV doesn't survive for very
long. In laboratory studies, the CDC has shown that once the
fluid (blood, sweat, tears, etc..) containing the HIV virus
dries, the risk of environmental transmission is nearly zero.
There is a lot of misinformation about how HIV can be
transmitted. So, here is a list of ways in which HIV is
Saliva, tears and sweat- Saliva and tears contain only small amounts of HIV,
and scientists haven't detected any HIV in the sweat of an
Insects - Studies show no evidence of HIV
transmission through bloodsucking insects. This is true even
in areas where there are many cases of AIDS and large
populations of mosquitoes.
In the next section, you will learn what happens once the
HIV virus enters the body, and how it attacks the immune
The Life-cycle of HIV Like all viruses,
HIV treads the fine line that separates living things from
nonliving things. Viruses lack the chemical machinery that
human cells utilize to support life. So, HIV requires a host
cell to stay alive and replicate. To replicate, the virus
creates new virus particles inside a host cell and those
particles carry the virus to new cells. Fortunately the virus
particles are fragile.
Viruses, like HIV, don't have cell walls or a nucleus.
Basically, viruses are made up of genetic instructions wrapped
inside a protective shell. An HIV virus particle, called a
virion, is spherical in shape and has a diameter of
about one 10,000th of a millimeter.
HIV infects one particular type of immune system cell. This
cell is called the CD4+T cell, also know as a T-helper cell
(see How the
Immune System Works for details on T cells). Once
infected, the T-helper cell turns into a HIV-replicating cell.
T-helper cells play a vital role in the body's immune
response. There are typically 1 million T-cells per one
milliliter of blood. HIV will slowly reduce the number of
T-cells until the person develops AIDS.
To understand how HIV infects the body, let's first look at
the virus's basic structure. Here are the basic parts of the
Viral envelope - This is the outer coat of the
virus. It is composed of two layers of fatty molecules,
called lipids. Embedded in the viral envelope are
proteins from the host cell. There are also about 72 copies
of Env protein, which protrudes from the envelope
surface. Env consists of a cap made of three or four
molecules called glycoprotein (gp) 120, and a stem
consisting of three to four gp41 molecules.
p17 protein - The HIV matrix protein that lies
between the envelope and core
Viral core - Inside the envelope is the core,
which contains 2,000 copies of the viral protein,
p24. These proteins surround two single strands of
HIV RNA, each containing a copy of the virus's nine genes.
Three of these genes -- gag, pol and env -- contain
information needed to make structural proteins for new
HIV is a retrovirus, which means it has genes
composed of ribonucleic acid (RNA) molecules. Like all
viruses, HIV replicates inside host cells. It's considered a
retrovirus because it uses an enzyme, reverse
transcriptase, to convert RNA into DNA.
Once the HIV virus enters the body, it heads for the
lymphoid tissues, where it finds T-helper cells. Let's look at
how the HIV virus infects immune system cells and replicates:
Binding - The HIV attaches to the immune cell
when the gp120 protein of the HIV virus binds with the CD4
protein of the T-helper cell. The viral core enters the
T-helper cell and the virion's protein membrane fuses with
the cell wall.
Reverse transcription - The viral enzyme, reverse
transcriptase, copies the virus's RNA into DNA.
Integration - The newly created DNA is carried
into the cell's nucleus by the enzyme, viral
integrase, and it binds with cell's DNA. HIV DNA is
called a provirus.
Transcription - The viral DNA in the nucleus
separates and creates messenger RNA (mRNA), using the
cell's own enzymes. The mRNA contains the instructions for
making new viral proteins.
Translation - The mRNA is carried back out of the
nucleus by the cell's enzymes. The virus then uses the
cell's natural protein-making mechanisms to make long chains
of viral proteins and enzymes.
Assembly - RNA and viral enzymes gather at the
edge of the cell. An enzyme, called protease, cuts
the polypeptides into viral proteins.
Budding - New HIV virus particles pinch out from
the cell membrane and break away with a piece of the cell
membrane surrounding them. This is how enveloped viruses
leave the cell. In this way, the host cell is not destroyed.
The newly replicated virions will infect other T-helper
cells and cause the person's T-helper cell count to slowly
dwindle. The lack of T-helper cells compromises the immune
system. When a person's T-helper cell count drops below
200,000 cells per one milliliter of blood, he or she is
considered to have AIDS. The development of AIDS takes about
two to 15 years, but about half of all people with HIV will
develop AIDS within 10 years after becoming infected,
according to the CDC.
No one dies from AIDS or HIV specifically. Instead, an
AIDS-infected person dies from infections, because his or her
immune system has been dissipated. An AIDS patient could die
from the common cold as easily as he or she could from cancer. The
person's body cannot fight off the infection, and he or she
World Impact To understand the devastation
of AIDS, you have to understand the high mortality rate of
people who develop the disease. If you counted every person in
the city of Chicago, which is about 3 million, you would get
the idea of how many people died worldwide from AIDS in 2000.
Basically, that means that each year AIDS kills the same
number of people that populate the third largest city in the
More then 36 million people are infected with the HIV virus
worldwide, with 25.3 million of those cases in sub-Saharan
Africa. Additionally, anther 5.3 million new HIV infections
occurred in 2000, which represents about 16,000 new cases per
day. The regions with the greatest number of people living
HIV/AIDS, according to the World Health Organization,
Sub-Saharan Africa - 25.3 million
South and Southeast Asia - 5.8 million
Latin America - 1.4 million
North America - 920,000
Eastern Europe/Central Asia - 700,000
In the United States, 753,907 cases had been reported to
the CDC through June 2000. However, the CDC estimates that as
many as 900,000 Americans are living with HIV or AIDS.
1926-46 - HIV possibly spreads from monkeys
to humans. No one knows for sure.
1959 - A man dies in Congo in what many
researchers say is the first proven AIDS death.
1981 - The Centers for Disease Control and
Prevention (CDC) notices high rate of otherwise rare
1982 - The term AIDS is used for the first
time, and CDC defines it.
1983/84 - American and French scientists
each claim discovery of the virus that will later be
1985 - The FDA approves the first HIV
antibody test for blood supplies.
1987 - AZT is the first anti-HIV drug
approved by the FDA.
1991 - Basketball star Magic Johnson
announces that he is HIV-positive.
1996 - FDA approves first protease
1999 - An estimated 650,000 to 900,000
Americans living with HIV/AIDS.
2001 - AIDS global death toll reaches
nearly 22 million.
AIDS is clearly one of the worst health crises facing the
world today. Without any truly effective treatment, most
health experts are putting an emphasis on prevention to stop
the spread of HIV. To learn more about HIV and AIDS, go to the