The individual host, a unique evolutionary island for rapidly dividing parasites: a theoretical approach.

It is hypothesized that rapidly dividing parasites producing high parasitemias within an individual host are in different environmental settings. It is further suggested that these infrapopulations experience the drastic environmental changes of free-living forms in an island environment and, that in chronically infected animals, the environmental conditions will over time select the parasites best suited to grow in their changing habitat. Evidence is presented to demonstrate that the host environment does change during an infection with the African trypanosomes and, that with time, each host becomes environmentally unique. Data are also provided to show that parasites cloned from different hosts are phenotypically different and are assumed to be genetically different as well. The evidence provided is consistent with the hypothesis that each individual host provides a unique habitat in which selection occurs, and that the rapidly dividing protozoans, such as the African trypanosomes and plasmodia are continuously evolving in the individual host.