Isaac Asimov

Four men and a woman are reduced to a microscopic fraction of their original size, sent in a miniaturized atomic sub through a dying man's carotid artery to destroy a blood clot in his brain. If they fail, the entire world will be doomed.

Far as Human Eye Could See is the 19th collection of science essays by Isaac Asimov, short works which originally appeared in The Magazine of Fantasy and Science Fiction, these being first published between November 1984 and March 1986.