proposed that HIV emerged

proposed that HIV emerged because of rapid serial human-to-human transmission of SIV (after a bushmeat hunter or handler became SIV-infected) through unsafe or unsterile injections.^{[16][18][33][34]} Although both Chitnis et al.^[27] and Sharp et al.^[7] also suggested that this may have been one of the major risk factors at play in HIV emergence (see above), Marx et al. enunciated the underlying mechanisms in greater detail, and wrote the first review of the injection campaigns made in colonial Africa.^[18][33]

Central to Marx et al. argument is the concept of adaptation by serial passage (or serial transmission): an adventitious virus (or other pathogen) can increase its biological adaptation to a new host species if it is rapidly transmitted between hosts, while each host is still in the acute infection period. This process favors the accumulation of adaptive mutations more rapidly, therefore increasing the odds that a better adapted viral variant will appear in the host before the immune system suppresses the virus.^[18] Such better adapted variant could then survive in the human host for longer than the short acute infection period, in high numbers (high viral load), which would grant it more possibilities of epidemic spread.

Marx et al. reported experiments of cross-species transfer of SIV in captive monkeys (some of which made by themselves), in which the use of serial passage helped to adapt SIV to the new monkey species after passage by three or four animals.^[18]

In agreement with this model is also the fact that, while both HIV-1 and HIV-2 attain substantial viral loads in the human organism, adventitious SIV infecting humans seldom does so: people with SIV antibodies often have very low or even undetectable SIV viral load.^[22] This suggests that both HIV-1 and HIV-2 are adapted to humans, and serial passage could have been the process responsible for it.

Marx et al. proposed that unsterile injections (that is, injections where the needle or syringe is reused without sterilization or cleaning between uses), which were likely very prevalent in Africa, during both the colonial period and afterwards, provided the mechanism of serial passage that permitted HIV to adapt to humans, therefore explaining why it emerged epidemically only in the 20^th century