Travel to Ballybran itself, however, is forbidden to all but its resident singers and supporting population, about 30,000 people. The moon-side orientation program secretly explains why: a native spore soon invades the human body and causes genetic mutations. Some newcomers will die of the initial infection and many will adapt only partly, with a mix of permanent symptoms such as vastly increased visual acuity along with complete deafness. Those who adapt fully to symbiotic life may become singers; other survivors must join the staff. The symbiont maintains its host, perhaps for hundreds of years, but only on Ballybran; only the fully adapted singers can safely depart, and only briefly.