Which theory on the origin of HIV seems the most viable?

HIV has had some crazy origin stories, from CIA conspiracies to God’s judgment, but the most likely origin of HIV is simian. The strains HIV-1 and HIV-2 have much in common with the simian immunodeficiency virus (SIV), and researchers estimate that the virus made eight leaps before becoming the HIV we know today. A recent study from Oxford University’s Zoology Department makes a very compelling case that the ancestor of HIV emerged in 1920 in what is now the Democratic Republic of Congo. Several factors, including a population boom, new transportation networks (HIV grew along with train routes), changing practices of sex workers, decolonization, and growth of using injectable medicine (and reuse of the needles) all contributed to the spread into the larger population during the 1960s.