 The study is a proof-of-concept clinical investigation designed to develop a safe and practical sporozoite challenge model for Plasmodium vivax in humans with a goal of 100 infectivity rate. The development and standardization of such a model will make possible efficacy evaluations of candidate P. vivax vaccines in Phase 2a trials. This trial is conducted in collaboration with Armed Forces Research Institute of Medical Sciences AFRIMS investigators in Bangkok, Thailand, who will be recruiting adult blood donors from a pool of patients who present with active P. vivax infections in Thailand. Samples of P. vivax infected blood will be collected and fed via membrane feeding apparatus to colony-reared Anopheles dirus mosquitoes at the AFRIMS Entomology Lab. A portion of the same blood will meanwhile be screened for potential co-infections at the AFRIMS Retrovirology Laboratory. When screening tests have confirmed the presence of only P. vivax in the blood no co-infections with other malaria species, and selected dissection on days 3-7 has revealed oocyst production in the blood-fed mosquitoes, the mosquitoes will be transported from the AFRIMS insectary in Thailand to the WRAIR insectary in the US by a standard procedure herein described including permits and assurance against accidental release of the infected mosquitoes. Transport will be conducted in compliance with Thai exporting and US importing requirements. 