Tick carrying Lyme disease bacteria, conceptual image. Computer illustration representing a tick carrying Borrelia burgdorferi bacteria (curled lines), the pathogen that causes Lyme disease. Ticks (superfamily Ixodoidea) are parasitic arachnids that feed on the blood of mammals, birds and reptiles. They are vectors for a number of animal and human diseases, including Lyme disease, which is characterised by early symptoms including fever, headache, fatigue, depression, and a circular skin rash called erythema migrans.