A 34-year-old man with HIV presents with a 3-month history of numerous skin lesions that have continued to spread despite avoiding close skin contact and using over-the-counter antibacterial washes. He reports no pain or pruritus. He has had unintentional weight loss and intermittent oral thrush. Physical examination shows dozens of 2–5 mm, flesh-colored, dome-shaped papules on the face, trunk, and groin; many have a central dell (umbilication). No inguinal lymphadenopathy is present. Which of the following best explains why this patient is likely to have a prolonged course with widespread lesions?