well, here is my theory:yeerk personalities are like anyother creature, the are a result of memories, learned experences, and the driving force (the thing that holds it altogether, it is also called the Soul).
but if a yeerk continually infests one type of person (not species but person) the yeerk starts to adopt (uncounisly) emotional traits of that host.
an example of this would be a yeerk who infests a host who has a overwheling love for an indiviual, will sooner or latter start feeling the same way. likewise, if a yeerk infests a host that is continually depressed, eventually the yeerk starts feel the same way.
the barriors between minds is a frailge thing and it is all to easy for one mind to (unknowingly) adopt the traits of another.
as for the topic it's self:
chances are we (the human race) would fight to the end and that end would problely end up being using the M.A.D. Staragy and nuking everyone and everything in process.