Most human cases are caused by the nymphal, or immature, form of the tick. Nymphs are about the size of a poppy seed. Because their bite is painless, many people do not realize they have been bitten.

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Ixodes ricinus has a three-host life cycle, which usually takes 2–3 years to complete, although it can take from 1 to 6 years in extreme cases.[3] Adults feed on large mammals such as sheep, cattle, dogs, deer, humans and horses for 6–13 days, before dropping off. An engorged female will lay several thousand eggs and subsequently die.[3] The larvae that hatch do not actively seek a host, and usually feed on insectivores (orders Erinaceomorpha and Soricomorpha), although they may also find rodents, rabbits, birds, reptiles or bats.[3][6] They feed for 3–5 days before dropping off and moulting. The resulting nymphs then ascend grasses or twigs to seek their next host, but must return to the moist microclimate at the soil surface if they become dehydrated.[7] The nymphs feed on small to medium-sized mammals.[5]

Ixodes ricinus is found across Europe and into neighbouring parts of North Africa and the Middle East, extending as far north as Iceland and as far east as parts of Russia.[3] Its northern limit seems to be determined by environmental factors, including temperature, since a series of mild winters in Scandinavia coincided with an expansion northwards in the range of I. ricinus.[4]

I. ricinus is most frequent in habitats where its hosts are plentiful, including woodlands, heaths and forests.[3] It is most prevalent in relatively humid areas, and is absent from much of the Mediterranean Region where summers are dry.[5