"None of the wicked will understand, but those who are wise will understand."
Daniel 12:10

One should note that some of the closest relatives of SIVcpz (HIV-1 group N) and of SIVsm (HIV-2 groups C through G) are extremely rare in humans, with only 6 HIV-1 group N-infected patients known and only single individuals infected by HIV-2 groups C-G. This suggests that cross-species transmittedviruses are not sufficiently adapted for spread into the new host population to generate an epidemic. This situation is not unique in virology: direct transmission of influenza from its avian host seems to have a lower pandemic potential than the transmission of recombinant influenza viruses originated from the pig's "mixing vessel": only 18 cases of  H5N1 influenza infection have been recorded during the epidemic in Hong Kong.

These cases were severe, with a mortality rate of more than 30%. However, no evidence of human-to-human transmission of H5N1 virus was found. Moreover, serological screening of poultry workers directly exposed to the avian virus has shown that about 10% were seropositive, and that the infection was asymptomatic or mildly symptomatic, withno secondary cases reported. These findings suggest the need for animal transmitted viruses to adapt to the new host before initiating significant epidemics or pandemics

2. Experimental cross-species transmission of SIVs in different species of monkeys has shown that in many cases the virus is harmless or cleared by the new host. Similarly, some of the HIV-2 groups show low pathogenic potential in the human host. These findings lead to the conclusion that only the zoonotic origin of HIV-1 is known, whereas the disease is a pure human nosological entity. Again, these data shows that in order to induce the disease, the virus must undergo adaptation and selection processes in the new host. There are no data to support AIDS as a zoonosis.

3. SIVs infections in their natural host are generally asymptomaticand immunodeficiency is extremely rare. This finding reinforces the assumption that a change in the pathological potential of the virus is needed for SIV to become pathogenic in the new primate host. In zoonotic infections such as rabies or West Nile encephalitis, the animal source is also susceptible to the disease. However, at least for rabies, it had been shown that the animal reservoir might be resistant to the disease.