They can wait for months, clinging to the edge of a blade of grass or a bush, for the whiff of an animal's breath or vibration telling them a host approaches.

They are ticks—and when they attach to your skin and feed on your blood over many days, they can transmit diseases. Often hard to diagnose and tricky to treat, tick-borne illnesses—led by Lyme disease—can cause symptoms ranging from headache and muscle aches, to serious and long-term complications that affect the brain,...