Pneumocystis carinii is a saprophyte of low virulence that primarily occurs in the mammalian lung. The taxonomy of P. carinii is uncertain: it has been classified as a unicellular protozoan as it is sensitive to drugs used to treat protozoan infections; its reproductive behavior is similar to that of yeast cells; and phylogenetic classification based on 16S-like rRNA sequences indicates that P. carinii is most closely related to a fungus. The morphology of the organism and the histopathology of the lesions produced by both human and animal isolates throughout the world are similar. Only a single species name has been assigned to the genus Pneumocystis, but antigenic differences suggest that several strains may exist. Although currently controversial, 4 species have been described: 2 species that infect dogs and rats, P. carinii and P. wakefieldiae; 1 species that infects mice P. murina; and P.jirvecii, which infects humans. Biologic differences between isolates from different hosts are suggested by the relative difficulty of experimental interspecies transmission.
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机译：Pneumocystis carinii是一种低毒力的嗜酸体，主要发生在哺乳动物肺中。 P. Carinii的分类是不确定的：它被归类为单细胞原生动物，因为它对用于治疗原生动物感染的药物敏感;其生殖行为类似于酵母细胞;基于16S样RRNA序列的系统发育分类表明P. CariniI与真菌最密切相关。世界各地的人类和动物分离株产生的生物体和组织病理学的形态相似。只分配了肺炎属的单个物种名称，但抗原差异表明可能存在几种菌株。虽然目前有争议，但已经描述了4种：2种感染狗和大鼠，P. Carinii和P.Wakefieldiae; 1种感染小鼠P. Murina的物种;和p.jirvecii，它感染人类。通过实验性差异传输的相对难度提出了来自不同宿主的分离株之间的生物学差异。
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