This study investigates the potential for biological repair in UV-irradiated oocysts, and it presents the results of concurrent lines of investigation to examine whether reactivation occurs following UV disinfection and also to determine if C. parvum has the genetic basis for repair. The primary objectives of the study included the following: Determine whether C. parvum oocysts of multiple isolates, irradiated with varying doses of UV light, can repair and regain infectivity under light or dark conditions using in-vitro cell culture with a human cell line. Confirm cell culture results with animal infectivity using a neonatal mouse model. Identify possible DNA repair genes in C. parvum by performing homology searches with sequence databases, polymerase chain reaction (PCR) assays, and sequencing clones from a genomic library. Measure the extent of UV-induced DNA damage by quantitative PCR. If reactivation is detected, evaluate the effects of both post-UV chemical disinfection on oocyst reactivation and determine the increased UV dosage necessary to inhibit repair. Originally published by AwwaRF for its subscribers in 2004.