Apicomplexa
Most members have a complex life-cycle, with both asexual and sexual reproduction. A host is infected by an active invasion by the parasites. The parasites divide to produce sporozoites which enter its cells.  Eventually, the cells burst, releasing merozoites, which infect new cells.  Eventually, gametes are produced, and they fuse to create new cysts.  There are many variations on this basic pattern, however, and many Apicomplexa have more than one host.