Actually, worldwide the estimate is that 50% of people have toxoplasmosis (30% in the U.S.).  And while most are fortunately asymptomatic at the present, I don't think you can say they have developed immunity.  The cysts are still alive in them, doing whatever they do, which at this point we don't fully understand.  And as this population ages, more and more will become immuno-compromised through cancer and other illnesses.  It is then that this parasite becomes life-threatening.  On top of that, there is the Alzheimer's connection, along with other mental disorders.