Babesia divergens builds a complex population structure composed of specific ratios of infected cells to ensure a prompt response to changing environmental conditions

Summary Babesia parasites cause a malaria-like febrile illness by infection of red blood cells (RBCs). Despite the growing importance of this tick-borne infection, its basic biology has been neglected. Using novel synchronization tools, the sequence of intra-erythrocytic events was followed from invasion through development and differentiation to egress. The dynamics of the parasite population were studied in culture, revealing for the first time, the complete array of morphological forms in a precursor–product relationship. Important chronological constants including Babesia's highly unusual variable intra-erythrocytic life cycle, the life span of each population of infected cells and the time required for the genesis of the different parasite stages were elucidated. Importantly, the maintenance of specific ratios of the infected RBC populations was shown to be responsible for the parasites' choice of developmental pathways, enabling swift responses to changing environmental conditions like availability of RBCs and nutrition. These results could impact the control of parasite proliferation and therefore disease.