 If all six volunteers in the first cohort develop P. vivax infection, the same procedure will be repeated in the second cohort. If the first cohort has less than 100 infectivity rate, the second cohort will be challenged with up to 10 grade 2 infective bites to ensure 100 infectivity rate. Volunteers will be closely monitored post-infection, and will be treated with standard chloroquine and primaquine therapies when the infection becomes patent in the peripheral blood. 