A system, method, apparatus and diagnostic test for Plasmodium vivax , to determine a likelihood of a specific timing of infection by P. vivax in a subject, and hence identify individuals with a high probability of being infected with otherwise undetectable liver-stage hypnozoites. The system, method, apparatus and diagnostic test relate to the identification of hypnozoites ("dormant" liver-stages), or at least of the likelihood of the subject being so infected. Optionally and preferably, the specific timing relates to recent infections, for example within the last 9 months.