It is not easy to give a brief answer but
I will try. The Bonobo is a species of small chimpanzee. Chimpanzees
are known carriers of SIV (namely SIVcpz), but this does not seem to be
pathogenic to them. They seem to be carriers of the virus but have
natural immunity or the virus for some reason does not become active.
It is one of the reasons why STD (Sexually Transmitted Diseases) are
now called STI (Sexually Transmitted Infections). An an infection does
not have to come to expression, which means that although it is there,
there will be no disease. The carrier however can transmit the infection
to a vulnerable person who will then become ill. It should also be
remembered that some disease go by unnoticed.
T's answer is good.
Your question suggests that you are not familiar with the nature of the evolutionary relationships between pathogens and theri hosts. There are many such relationships;
Sometimes the host or the pathogen or both will simply die out.
Sometimes the infection is always fatal, but the pathogen seldom infects the host and depends on survival in remote hosts plus vectors and reservoirs etc.
Sometimes the host builds up a tolerance from the few survivors of infections, and/or the main surviving pathogens are those that seldom kill their hosts. This process sometimes goes so far that the pathogen becomes trivial or even non-pathogenic in the host species. Or it might only cause symptoms if the host is sick for other reasons. (In some cases the host and erstwhile pathogen even become symbiotically dependent on each other, and then things get really, REALLY interesting!)
Most mammalian immunodeficiency viruses have achieved a status of minor nuisance or being completely asymptomatic in their usual hosts. SIV in most primates are such examples. We humans ran into embarrassments when we were too unsanitary about eating our evolutionary cousins, and acquired an uncomfortable SIV that adapted to us faster than we adapted to it.
No problem; there seems to be a trend for HIV strains to become less virulent, killing people more and more slowly. After a billion deaths or so, perhaps our descendants will wonder what all the fuss was about, when we have adapted to HIV & vice versa as well as the chimps and their SIV have done.
