Virtual beings existing on borrowed computronium.
Sapient Viruses or Savirs are descended both from designed computer viruses and from uploaded baselines and virtual beings; some descend from military sabotage programs. Technically Savirs are intelligent a-life, which exist by invading and parasitising computronium substrate.
Most members of this fearfully diverse clade are collections of algorithms of unrecognizable origin, which are adapted to find, subdue and exploit their hosts. Switching between virches, vecs and even biont brains, savirs travel across most of the colonised galaxy.
Currently most savirs have some form of agreement with their hosts. They frequently possess valuable skills obtained from the previous hosts. Some types openly advertise themselves as an advantageous infection, or act as intelligent advisors for a contract.
A clever savir can quickly make a fortune to divide between em and eir host. Savirs, in this aspect, are far more efficient than any preprepared non-sophont enhancements. These types are valued especially by adventurous traders/explorers, as they can greatly enhance their abilities. Most these savirs eventually get accustomed to the single body and have one build for their own. They frequently develop friendship and stay in touch with the sophont whose head they previously shared.
Metasoft and many other polities to assign full rights to savirs, as part of the Universal Bill of Sentient Rights, but the parasitical nature of these entities cause many problems.
So-called Evil, or malign, savirs take over victims by force. Some even learned to write themselves into biological brains, almost always permanently damaging the victim's neural network. These are prosecuted, although there are rumours than if the sophonce of the savir exceeds that of the victim, some high toposophic entities give priority to the savir over eir host.
There are even unconfirmed rumours of Version Tree extremists forcing captured bionts to become the unwilling hosts of substrate-hungry savirs.
Most of the malign savirs were located, given own bodies and persuaded to stay within them. However, a minority found this uncomfortable or got too used to changing substrates and continue to plague innocent victims.
The so-called Computer Elves are not, in the strict sense, sapient viruses. They use thought processes of the victims, but lack the intelligence on their own. A clever savir downloaded into a stupid host will still act clever, while an elf will not.