Just 20 years ago, we'd never heard of the disease we now know as AIDS. Even when it first made its appearance in 1981, we didn't know it was AIDS. It was simply a mystery disease, affecting gay men, that seemed to destroy the immune system. Some people called it GRID - gay related immune disorder.

However, it wasn't long before doctors realised that this strange new disease affected everyone. Within a year, women and children were beginning to get it. That's when it first became known as AIDS, short for Acquired Immune Deficiency Syndrome.

The major feature of the disease is the destruction of the immune system, which is normally the body's way of fighting disease.Without an immune system, even a small cut can become so infected you can die from it. The common cold becomes potentially lethal. The body has no way of overcoming the germs and getting rid of them.

But what causes this strange state of affairs? In 1982, scientists were equally puzzled. The disease had a name, and that was all they knew about it.

In 1983, French scientists isolated a virus, and in 1984 American scientists isolated another. In fact, they had found the same virus, but they only discovered that two years later. So it was only in 1986 that the virus gots its name HIV, short for Human Immunodeficiency Virus.

And what it does is actively wipe out the cells that form the body's immune system, resulting in the disease known as AIDS.

It binds with a special immune cell called a CD4 T-lymphocyte, and plants its own viral code into the cell's nucleus. Then, when the immune system calls for more immune cells to fight an infection, the CD4 cells copy the virus instead of themselves.

A healthy person normally has between 1000 and 1200 CD4 cells in a cubic millimetre of blood. Soon after infection with HIV, that number drops dramatically. However, at this early stage, the body still has enough uninfected CD4 cells to fight back and bring the HIV infection under control. The infected person get all the symptoms of a bad cold or 'flu. But that person doesn't yet have AIDS. They're simply infected with HIV.

However, the number of CD4 cells never climbs back to where it was before infection. The body manages to rebuild a fair number of healthy immune cells, but not quite enough.

Over time, the amount of virus in the blood increases, and the number of CD4 cells drops and drops. Once it hits a certain critical number, the body becomes vulnerable to certain diseases that are normally very rare. These are the two important issues that determine when a person has AIDS.

If the number of CD4 cells drops below 200 per cubic millimetre of blood, that person officially has AIDS.

Or, if the person is HIV positive, and develops one of the AIDS-defining diseases, that person also has AIDS, even if the number of CD4 cells is still quite high.

There are people who have been HIV positive for nearly twenty years without developing AIDS. There are also people who develop AIDS within a couple of months of HIV infection.

It probably has something to do with how strong their immune systems were to begin with.

The AIDS-defining diseases are those which are incredibly rare in the general population. Most of them are caused by a fungus or bacterium which lives in the body all the time, normally quite harmlessly. When the immune system breaks down completely, these fungi or bacteria can suddenly thrive and infest the body.

The most important of these is a rare pneumonia known as Pneumocystis carinii pneumonia, or PCP for short. Before 1981, maybe 107 cases were reported in the US. Then suddenly there were 5 cases in one community in one year. PCP was the disease that alerted the Center for Disease Control that something strange was happening in the gay community, and the first signs of the new mystery illness. Today, PCP is common in AIDS patients, and still very rare in the general uninfected population.

Mycobacterium Avium Complex, or MAC for short, is another AIDS-defining disease. This one is caused by bacteria related to tuberculosis. Scientists think it is transmitted in soil or water, but no one knows for sure. However, the bacterium doesn't normally cause illness, except in the absence of an immune system. Before 1984, the US medical records knew of only 37 cases. Now, most AIDS patients with CD4 counts of less than 50 will get the disease.

Of course, AIDs patients get diseases which affect the general population too. They just get them worse and often have different symptoms. Candidiasis and herpes are good examples of these. In the general population, women often get itchy vaginal infections caused by candida. AIDS patients suffer from candida overgrowth in their throats and lungs. Over 70% of the population has a latent herpes infection, which can cause cold sores and fever blisters when activated. In AIDS patients, the virus causing herpes can be activated more often, and the blisters that form can become infected by bacterial or fungal overgrowth.

The good news is that new therapies are making an impact on the AIDS-defining diseases and opportunistic infections. The new drug combinations that help keep the HIV virus in check are also keeping the number of AIDS-defining diseases in check. They not only keep levels of virus in the blood very low, they also help prevent the number of CD4 cells from dropping.

They're slowing down the destruction of the immune system and making the time period from HIV infection to AIDS longer.

In short, HIV and AIDS are linked, but not the same thing. HIV is a virus which destroys immune system cells. AIDS is a collection of symptoms which define the lack of an immune system. You can have HIV without having AIDS. We haven't yet found AIDS without HIV.