 More than 75% of children are seropositive by the age of 4, which suggests a high background exposure to the organism.A " post mortem" study conducted in Chile of 96 persons who died of unrelated causes (suicide, traffic accidents, and so forth) found that 65 (68%) of them had pneumocystis in their lungs, which suggests that asymptomatic pneumocystis infection is extremely common."P.