Using laboratory-bred natural rodent hosts that had been castrated and then implanted with either testosterone or inert oil, we have shown that testosterone causes prolonged and more intense infections of a tick-borne piroplasm, Babesia microti. This will result in more ticks becoming infected while feeding. Sexually active male rodents with high testosterone levels are also known to show increased locomotory activity and reduced innate and acquired resistance to tick feeding, so that more ticks are likely to be picked up and then fed successfully by these hosts. As a result, the transmission potential of B. microti is significantly increased via hosts with high rather than low testosterone levels. It is argued that testosterone helps to generate the observed aggregated distributions of parasites amongst their hosts, which also enhances parasite persistence.