So far we have mentioned only one kind of pathogen adapta- tion, the ability to nourish itself in the body of the host. We can also expect it to have evolved ways of shielding itself from the host’s efforts to destroy, expel, or sequester it. We will now turn to one such mechanism, evasion of host defenses. The first trick for many parasites, once inside the body, is to gain entrance to cells. Invaders may accomplish this just as door-to-door peddlers do, by appearing to offer something else. The rabies virus binds to acetylcholine receptors as if it were a useful neurotransmit- ter; the cowpox virus to epidermal growth-factor receptors as if it were a hormone; and the Epstein-Barr virus (which causes mononu- cleosis) to a C4 receptor. Rhinovirus, a common cause of colds, binds to the intercellular adhesion molecule (ICAM) on the surface of the lymphocytes that line the respiratory tract. This is extremely clever, since attacking lymphocytes releases chemicals that greatly increase the number of ICAM binding sites, thus providing many more openings by which the virus can enter cells.