Pneumocystis "carinii" is actually a name that represents a complex of morphologically similar, but genetically distinct, species found in a wide range of hosts. The species infecting the lungs of humans has been termed P. jiroveci Frenkel, 1999 whereas those from most other hosts (and in most cases humans too) are collectively termed "Pneumocystis carinii." The original host from which the organism was first described was a guinea pig. The species above is from the lung of a rat.