The Hets, more properly known as Hhhet, are a species of sentient virus native to Mars. They infect larger animals and enslave them as hosts, which they use to reproduce, move, and interact with the world around them.
Anatomy and physiology
Individual Het are viral particles a few dozen angstroms across, small enough to move between individual cells. These particles consist solely of DNA, RNA, and protein, and by themselves are not intelligent. However, in large masses, the viruses form "synapses" that allow for collective thought. The larger the viral mass, the more intelligent the Het. The typical Het encountered in End of an Era masses about one or two kilograms. These viral masses are amorphous, gelatinous blobs, sky-blue in color and slightly phosphorescent.
As viruses, the Het do not "eat." They instead rely on the metabolic processes of their hosts for most functions. How they move outside of a host is never specified. However, it is likely that they use ATP as some real viruses do, or that they use the electricity generated by their "synapses."
Like all viruses, Hets do not reproduce on their own. They instead hijack the cells of true organisms in order to assemble more viruses.
The Het are capable of building highly advanced machinery using their host bodies, but prefer to use biotechnology that they can control directly rather than machines controlled by potentially rebellious slaves. For this reason, virtually all Het technology is biological in nature, and even their spacecraft are living organisms.
The Het have also discovered how to manipulate gravity. They propel their spacecraft by "polarizing" gravity, and even managed to decrease the gravity of Earth to that of Mars. Het spacecraft are incapable of faster-than-light travel, and they are limited to the solar system.