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.