HIV stands for Human Immunodeficiency Virus, a pathogen that works by attacking the human immune system. It belongs to a class of viruses called retroviruses and more specifically, a subgroup called lentiviruses, or viruses that cause disease slowly. 

HIV cannot replicate on its own, so in order to make new copies of itself, it must infect cells of the human immune system, called CD4 cells. CD4 cells are white blood cells that play a central role in responding to infections in the body. 

Over time, CD4 cells are killed by HIV and the body’s ability to recognise and fight some types of infection begins to decline. If HIV is not controlled by treatment, the loss of CD4 cells leads to the development of serious illnesses, or ‘opportunistic infections’. In people with normal CD4 cell levels, these infections would be recognised and cleared by the immune system. 

Experiencing a collection of these infections is the most advanced stage of HIV, which is when a person is also said to have AIDS (Acquired Immune Deficiency Syndrome). Effective testing and treatment of HIV means that the large majority of people living with HIV do not reach this stage.