question:  What is the true cause of Earth’s “plague” and what is its purpose?
context: New York lay quietly below him. It was the hour of indecision when day has not quite made up its mind to leave and night has not yet attacked in force. On sudden impulse he switched it on, turned the volume up full, and in grim jest left it playing Rachmaninoff's Isle of the Dead on full automatic. Buildings were taller, men were healthier, most of the problems of humanity seemed licked, or nearly so. It was a time of free power, each small unit of population, each section of town operating on perpetual, ever-lasting, automatic atomic piles. Sales of poison dropped, scientific laboratories chained to a perpetual rodent-cycle began to complain bitterly. The largest cotton crop ever was forecast and rumors from Mexico had it that no one had died from scorpion bite in several weeks. A month later the meat animals, the birds and the household pets began dropping as rapidly as the flies which had dropped earlier. Within a year it was obvious to everyone that man was the only animal left on earth. The panic which had begun with the death of the animals was quieted somewhat by the fact that humans seemed immune to the pandemic. But the lakes full of dead fish caused a great stink and residents along the coasts began to move inland. Then just one year ago, the first human became infected with the strange malady. Within six months, half of the world's population was gone. Less than a month ago no more than a few thousand people remained in New York. Before the plague the Bureau of Vital Statistics had been one of man's crowning achievements. Housed as it was in a huge metallic globe of a building, it contained computers which kept exact account of every human on earth. Once a year the Bureau issued The Index, an exact accounting of Earth's four billion inhabitants. Only once, before the plague, had he seen the interior of this room. And again at the age of 10 each child returned to the Bureau for a recheck.
output:  The plague was facilitated by aliens, described as invisible, ovular beings. Their purpose is to move from planet to planet exterminating living systems.
options:  The plague was facilitated by aliens, described as invisible, ovular beings. Their purpose is to clear Earth of all life and start their own colony.  The plague’s true cause is never revealed. Just as Charles suspects at the time of his death, the fall of the human race is completely unreasonable and meaningless.  The plague was facilitated by aliens, described as invisible, ovular beings. Their purpose is to exterminate all of Earth’s life in order to start their own planetary garden.  The plague was facilitated by aliens, described as invisible, ovular beings. Their purpose is to move from planet to planet exterminating living systems.