Pneumocystis is an unusual fungus that is an opportunistic pathogen, causing an asymptomatic or mild infection in the normal host but pneumonia (PcP) in immunocompromised hosts. Untreated, the mortality rate from PcP approaches 100%. Even with treatment, mortality rates approach 10–20%. It is a ubiquitous organism infecting a wide array of mammalian species. Although the reservoir of infection for Pneumocystis has not been defined, direct airborne transmission from host to host has been proven under experimental conditions in rats.
The study of Pneumocystis has been problematic due to the inability to cultivate the organism or manipulate its cellular or molecular characteristics. As recently as the 1970s, a student studying Pneumocystis would have come away with the following understanding of its basic biology and function as a pathogen: Pneumocystis is an organism of low virulence found in many mammalian species. In humans, Pneumocystis pneumonia is a zoonosis resulting from reactivation of a latent infection acquired early in life. This concept of Pneumocystis arose largely through analogy to existing knowledge about other organisms to explain clinical observations, rather than through direct experimentation on the organism.
Over the past 25 years, we have learned more about Pneumocystis through controlled studies that have corrected some of the misconceptions contained in the “old” concept of Pneumocystis contagion stated above. This article is a brief summary of key research observations that give us a better, yet still incomplete, understanding of how Pneumocystis maintains its existence as an opportunistic pathogen.