This is a fantastic simulation!
With my first run, the Bibites and a viral strain seem to have evolved a host-parasite relationship. The Bibites neural map is now set up to repress their immune system if infected and other Bibites are nearby. Likewise, the viral strain has evolved to be harmless to the Bibites, as the only thing it does is weaken their immune system further.
I guess the evolutionary benefit here is that, with every Bibite infected by the harmless virus, other more harmful viral strains do not have any room left to infect the remaining Bibites. In turn, the virus is guaranteed to live on through the Bibites' offspring.