Because of the serious public health implications of CQ-resistant P. vivax in the Americas, it is critically important to limit reports to well-confirmed cases. In most cases, this will require measurement of CQ blood levels and genotyping of parasites from the initial infection and any suspected recrudescence. P. falciparum in vivo trials take advantage of well-established molecular markers that help differentiate cases of recrudescence and reinfection, by polymerase chain reaction (PCR) techniques. Although no universally accepted technique for this purpose exists for P. vivax, investigators plan to use the microsatellites, base pair repeats in the parasite genome, described by Imwong et al (Imwong, Sudimack et al. 2006). Investigators believe that having PCR-corrected analysis is especially important in the context of the long follow-up period, six months, investigators aim for this study.