“Disgusting, parasitic animals” —that was Aristotle’s appraisal of ticks, and few people since have rushed to disagree. Ticks live by drinking their host’s blood, siphoned through a beaklike appendage known as the hypostome. These intimate meals might be merely an irritation for humans were it not for the tick’s habit of paying its tab with fever-inducing pathogens. AVERAGE SIZE: 0.10 inch (2.5 millimeters) unengorged.