Ticks are haematophagous parasites related to spiders and mites. Their life cycle consists of a passive stage (egg) and three active stages: larva, nymph and adult. For development to the next stage, the tick requires a blood meal. Some ticks are highly host specific and feed on a limited number of animal species. Other ticks, including the sheep tick (Ixodes ricinus), feed on a wide range of hosts, such as large mammals, rodents and birds. Humans are usually incidental hosts.

The Ixodes ricinus tick (or sheep tick) is the most common tick in humans in Belgium. Studies conducted in 2017 and 2021 estimated that it accounts for 99% of the ticks through which a human is bitten. Other, more rare, tick species also occur.