This paper describes three stages of the deer ked (Lipoptena cervi) attack on humans—a victim that the deer ked mistakes for its usual hosts, i.e., forest mammals. The work combines field research, genetic studies, a literature review, and observations in forests around Iława (Poland). It details how the deer ked locates its victim by sight, then corrects its course using thermal sensors that detect warm places, and subsequently moves around on the host to select a site for piercing. The paper also examines the presence in the deer ked genome of traditional thermoreceptor genes as well as those potentially aiding in processing information about external temperature.