HIV/AIDS is the world's most effective virus.
To date it has claimed more than 27 million lives.
The HIV virus is spread through the exchange of bodily fluid
HIV - Human Immunodeficiency Virus
AIDS is the advanced stage of HIV infection.
AIDS - Acquired Immunodeficiency Syndrome
And it can take over 15 years for HIV to develop into AIDS.
HIV attacks the immune system - your body's defense system, infecting lymphocytes.
These are a type of white blood cell in the body's immune system that are supposed to fight off invading germs.
HIV targets specific lymphocytes called T helper cells, or T-cells.
These act as the host that the virus needs in order to replicate.
Many viruses, such as HIV, rely on RNA to carry their genetic material.
RNA - Ribonucleic acid
And have the unique ability to interfere with the host cells' DNA, hijacking it to produce virus DNA.
This ability to interfere with the host cells' DNA defines HIV as a retrovirus.
Retrovirus - host cell used to produce virus DNA
Dr Robert Gallo, Institute of Virology, Baltimore - "When this cell divides to become daughter cells, the DNA will be transmitted, of the virus, into each daughter cell. That means not only is this cell, the target cell, infected forever, but very likely the individual..."
Once infected, instead of performing normal immune functions, the T-cell will start producing new virus cells.
But it is not just the virus's ability to quickly multiply that makes it so deadly.
When an HIV-infected cell duplicates, the virus can mutate, changing its DNA.
This means that the immune system is fighting an invader that is constantly evolving.
Prof Wendy Barclay, Imperial College, London - "It's a very quick and dirty strategy viruses like to use in order to be able to change rapidly in response to things we throw at them like drugs and vaccines."
Prof Geoffrey Smith, Imperial College, London - "... so that if we did make a vaccine against one particular strain of HIV, there would be so many others appearing rapidly, that the vaccine really would not be effective."
The hunt for a successful vaccine for HIV/AIDS, one of the biggest viral killers of the modern age, remains one of science's major challenges.