<qa>  How many sporozoites are in salivary glands?Our study will involve two cohorts, each to be challenged once, in the hope of demonstrating reproducibility of the entire challenge procedure. Each cohort comprises up to 6 healthy adult volunteers. The first cohort will be challenged with 5 bites from P. vivax-infected mosquitoes each carrying at least a grade 2 sporozoite infection 10 sporozoites in salivary gland. 