The vector virus created by Zobrist, as described on p. 438,
causes a randomly selected one third of the human population
to become sterile. But how can a virus act randomly? If the
virus is inserted into the human germ-line, it will be faithfully
copied into all offspring with the precision of human DNA
replication, so variation in the viral genome, once incorporated
into the germ-line, is not possible. The only other way the virus
could affect only a third of the population is that there is some
other genetic property which enables the virus to render the
organism carrying it sterile. But if that is the case, and the
genetic property be heritable, only those who lacked the
variation(s) which allowed the virus to sterilise them would reproduce,
and in a couple of generations the virus, while still incorporated
in the human genome, would have no effect on the rate of growth
of the human population: “life finds a way”.
Further, let's assume the virus could, somehow, randomly
sterilise a third of the human population, that natural
selection could not render it ineffective, and science
found no way to reverse it or was restrained from pursuing
a remedy by policy makers. Well, then, you'd have a world
in which some fraction of couples could have children
and the balance could not. (The distribution depends upon
whether the virus affects the fertility of males, females, or
both.) Society adapts to such circumstances. Would not
the fertile majority increase their fertility to meet
market demand for adoption by infertile couples?