 Nine years later (1951), Dr. Josef Vanek at Charles University in Prague, Czechoslovakia, showed in a study of lung sections from 16 children that the organism labelled ""P. carinii"" was the causative agent of pneumonia in these children.The following year, Jírovec reported ""P. carinii"" as the cause of interstitial pneumonia in neonates.Following the realization that "Pneumocystis" from humans could not infect experimental animals such as rats, and that the rat form of "Pneumocystis" differed physiologically and had different antigenic properties, Frenkel was the first to recognize the human pathogen as a distinct species.