THE BIOLOGY OF AIDS

The HIV virus attacking an immune cell
(BBC NEWS)







INTRODUCTION

At least 28 million people worldwide have died from Aids – their bodies’ defence systems ravaged by the HIV virus to the point where everyday infections become life-threatening.
More than 20 years since HIV was first recognised, there remains no vaccine against HIV and no cure for Aids, although a new generation of drugs has dramatically extended the life expectancy of those who contract HIV.

1 – Virus attaches: Proteins on the HIV virus “dock” with CD4 receptors on the target cell.

2 – Genes copied: The HIV virus makes a copy of its own genetic material.

3 – Replication: The virus inserts this copy into the host cell’s DNA. When the cell reproduces, it manufactures the parts of the HIV virus.

4 – Release: The parts are assembled and form a “bud”, which breaks off to become a new HIV virus.

HIV – the Human Immunodeficiency Virus – targets the immune system, the very system which would normally defend the body against infections.

The virus attacks a particular type of white blood cells called CD4+ cells. It hijacks the cell, inserts its own genes into the cell’s DNA and uses it to manufacture more virus particles. These go on to infect other cells.

The CD4+ host cells eventually die, although scientists do not know exactly how.
The body’s ability to fight diseases decreases as the number of CD4+ cells drops, until it reaches a critical point at which the patient is said to have Aids – Acquired Immune Deficiency Syndrome.

HIV is a particular kind of virus – a retrovirus. While simpler than ordinary viruses, retroviruses tend to be harder to defeat.

They embed their genes into the DNA of the cells they target, so that any new cells that the host cell produces also contain the virus genes.

Retroviruses also copy their genes into the target cell with a high level of error. In combination with HIV’s high replication rate, this means the virus mutates at speed as it spreads.

Furthermore, the “envelope” the HIV virus particle is contained inside is made of the same material as some human cells, making it difficult for the immune system to distinguish between virus particles and healthy cells.

The HIV virus – present in blood and sexual fluids








HIV myths
HIV cannot be caught:

1 – through the air, or by coughing and sneezing
2 – by kissing, touching or shaking hands
3 – by sharing crockery or cutlery
4 – through contact with toilet seats
5 – through insect or animal bites
6 – through swimming pools
7 – by eating food prepared by someone with HIV

HIV is present in the blood, sexual fluids and breast milk of people who are infected with the virus.
It is passed on when these infected fluids get into another person’s system.

You can contract HIV by:

1 – Having unprotected sexual intercourse with someone who is infected
2 – Sharing needles or body piercing equipment with someone who is infected
3 – Being given a transfusion of infected blood
4 – Allowing infected fluids to get into a cut or sore anywhere on your body
5 – Babies born to HIV positive women can be infected during pregnancy and birth, or through breast feeding
6 – HIV is present in the saliva of an infected person, but not in quantities sufficient to transmit infection. Once infected fluids have dried, the risk of them transmitting the virus is considered to be close to zero.
7 – The key methods of preventing the transmission of HIV are by not having sex with anyone who is, or may be, HIV positive, or by using latex condoms.
8 – Latex condoms are essentially impermeable to HIV-sized particles. If used properly and consistently, they are considered highly effective in reducing the risk of transmission – although no protective method other than abstinence is 100% safe.
9 – Injecting drug users can reduce the risk of contracting HIV by not sharing needles.

THE PROGRESSION OF HIV

About half of people who contract HIV suffer flu-like symptoms within the first two to four weeks of infection.

These include fevers, fatigue and rashes, sore joints, headaches and swollen lymph nodes.
The graph on the right shows the course of a typical HIV infection over time. The CD4+ cell count is the number of CD4+ cells per cubic millimetre of blood, and decreases as the virus progresses.

A healthy immune system has 600 – 1200 cells per cubic millimetre of blood. If this drops as low as 200, the patient is considered to have Aids.
The “viral load” is the number of virus particles per millilitre of blood. Initially, this peaks as the virus replicates rapidly in the bloodstream.

Within six to twelve weeks of infection, the body starts producing a specific type of antibody, or disease-fighting protein.

While not very effective in fighting the virus, the antibody is a reliable indicator of whether someone is infected.

The most common HIV test detects the antibodies. This means that a person may infect others as soon as he or she becomes infected, but will not test positive for several weeks.

Some people with HIV may live for several years before developing Aids, feeling healthy and with no outward signs of the virus.

Others may suffer symptoms such as weight loss, fevers and sweats, frequent yeast infections, rashes and short-term memory loss while living with HIV.


INFECTION RISKS AS CD4 BLOOD COUNT DROPS

Click on the graph to read about some of the most common infections which affect HIV patients
*
Guide to key HIV terms (… like “CD4 count”)

As the immune system becomes damaged, it loses its capacity to fight disease and infections can become life-threatening.

People who are HIV positive are more susceptible to widespread diseases such as tuberculosis, malaria, pneumonia and shingles. Their vulnerability increases as their CD4+ cell count drops.
HIV patients also become vulnerable to a host of “opportunistic infections”. These are infections caused by common bacteria, funguses and parasites which healthy bodies can fight, but which can cause illness and in some cases death in people with damaged immune systems.

Some of these are likely to occur at higher CD4+ counts than others. Most become active below CD4+ counts of 200, the point where Aids develops.

If full medical care is available, patients can be given drugs which treat and guard against some of these infections, although these are sometimes expensive and can cause side effects.
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