Re: Where did mononucleosis originate?

Infectious mononucleosis is caused by Epstein-Barr Virus, which is 
a member of the human herpesvirus family.   It is one of the 
gammaherpesviruses.   Most viruses, including the herpesviruses,
are usually specific to a single species of host, but can sometimes
cross the species barrier to infect a new host.  For example, the
T-Cell leukemia viruses are present in many mammals, from mice
to non-human primates to humans, and it is evident that Human T-Cell
Leukemia Virus type I (HTLV-I) and Human T-Cell Leukemia Virus type II
(HTLV-II) represent at least two different transfers of this type
of virus from non-human primates to humans within that past 200,000
years or so.

It is not yet known when or where the Epstein-Barr Virus type 1
nor the Epstein-Barr Virus type 2 first began infecting humans.
Viruses don't leave fossils or other evidence from the distant
past, so studying their origins requires studying the genetic
sequences of related viruses from many other species of host, to
find out what other hosts have viruses most closely related to the
human virus, and then estimating how much time might have passed
since the two viruses last shared a common ancestor.  We do not
yet have isolates of the gammaherpesviruses from dozens of different
non-human primate species, so we can't yet tell when nor where
they began infecting humans.

Most viruses cross the host species barrier only between closely
related hosts.  For example from non-human primate to human, but
not from dog to human or dog to cat.  One exception to this is
the influenza viruses, which live in birds, pigs and humans.  The
bird and pig influenza viruses has never been observed to directly 
infect humans and start a new human epidemic form, but it is evident
that at least some of the genes from these flu viruses can move between
species on rather rare occasions.   Another example is the Hantavirus
which normally lives in mice but can infect humans.  The human
infection does not spread from human to human, so that the Hantavirus
never becomes what we would consider a human virus.

The Epstein-Barr virus is known to spread from human to human, and
it is considered a human virus.  It is possible that this virus has
been using humans as its host for thousand of years or even 
hundreds of thousands of years.