One person on a hike is “eaten alive” by mosquitos while his companions are not bothered at all. Do ectoparasites such as mosquitos, ticks, fleas, and mites bite randomly?

No. Most arthropods are attracted to their hosts by a number of physical and chemical stimuli such as warmth, composition of sweat, carbon dioxide, vibrations, and odor. On the other hand, lipids derived from the degenerative epidermal cells often have a repellent quality to them. Thus, to arthropods, some human hosts are far more attractive than others.