For readers unfamiliar with virology, this section reviews a few concepts, which are summarised in the table. HIV-1 and HIV-2 are the two human ‘retroviruses’ that cause AIDS. To replicate, retroviruses transcribe RNA into DNA, which is then integrated into the DNA genome of the human host cells. This process is the reverse of what normally happens (DNA transcripted into RNA), hence their name. Retroviruses are further subdivided into ‘lentiviruses’ (HIV-1 and HIV-2), ‘oncoviruses’ (HTLV-I, the first retrovirus isolated from humans, which does not cause AIDS but sometimes cancer or paraplegia) and ‘spumaviruses’ (not pathogenic).

Of course, the pandemic is caused by HIV-1. HIV-2 differed enough, genetically, from HIV-1 to be considered a distinct virus. It remained confined to West Africa, is less transmissible and less pathogenic than HIV-1 and slowly disappeared while HIV-1 spread throughout the world. HIV-1 infection is 100 times more common than HIV-2; when authors use the term HIV, in practice it generally means HIV-1.