Watch the water.

On the first page Stewart tells readers how contagion could bring the end very quickly for mankind:

"If a killing type of virus strain should suddenly arise by mutation…it could, because of the rapid transportation in which we indulge nowadays, be carried to the far corners of the earth and cause the deaths of millions of people." W. M. Stanley, in Chemical and Engineering News, December 22, 1947.[10]

Within a few pages he makes it clear that basic biology applies to humans too:

"Some zoologists have even suggested a biological law: that the number of individuals in a species never remains constant, but always rises and falls—the higher the animal and the slower its breeding-rate, the longer its period of fluctuation[…]As for man, there is littler reason to think that he can in the long run escape the fate of other creatures, and if there is a biological law of flux and reflux, his situation is now a highly perilous one….Biologically, man has for too long a time been rolling an uninterrupted run of sevens."