Perhaps people that do know they have Toxoplasmosis, know for sure; but many people have it do not know. So maybe the risk factor here isn't having Toxoplasmos

Perhaps people that do know they have Toxoplasmosis, know for sure; but many people have it do not know. So maybe the risk factor here isn't having Toxoplasmosis, but knowing you have Toxoplasmosis :-) (or more seriously: some other factor correlated with it, like being a veterinary or animal handler which may be required to test regularly)