Mankind is not alone in the universe. In January of 1993, man received instructions from an unknown alien intelligence telling how to create a combined alien/human DNA sequence. Ten human embryos were injected with the DNA sequence, and one was allowed to develop. This creature, code name SIL, grew at an astonishing speed. A decision was made to terminate the project, but SIL escaped the lab, after which the creature matured and took on the form of a fully grown human female. A team was assembled to track down this woman and eliminate her. Sil was driven by an overwhelming desire to mate. After succeeding to do so, she bore a child in an amazingly short period of time. Soon after the birth, both Sil and her child were destroyed. Unfortunately, the alien DNA infected a rodent and survived. What became of this alien is unknown.

In an effort to study the aliens and defend against them, another embryo was allowed to develop. This project was simultaneous with man's first steps on Mars. A trace of alien DNA was accidentally picked up in a soil sample. This DNA went through the rapid growth typically associated with these beings and broke free. It proceeded to infect two of the three crew members. Once they returned to Earth, they were discovered to be infected, and one crew member was exterminated before she could cause much damage. However, the other infected member, Patrick Ross, escaped and began to breed.

It was discovered that the second Sil, named Eve, had a psychic connection with Patrick. By exploiting this, a team was able to track down Patrick and his children. Eve and Patrick succeeded in mating before Patrick killed her. He was then destroyed using human blood with genetic defects (sickle cell anemia). Once again, not all of the aliens were destroyed, and one of Patrick's children survived. Also, the child of the union between Eve and Patrick ripped its way from the corpse of its mother. The fates of these beings are unknown.

The origin of this alien species is unknown. But DNA traces of it still exist on Mars from millions of years past when it destroyed the then thriving planet and left it dead. If brought into the proper environment, this DNA will develop into a full Sil. Each cell of the alien's body contains its entire genetic code, which can be injected into the cells of other beings like a super virus. It can replicate at an amazing speed, and it can take over a human-sized host in a few minutes. Once in a host, a Sil has a variety of forms. Its natural form is a writhing mass of tentacles. It can also retain the appearance of its host; however, to a degree, the alien can alter this form at will. This altered state has a thick carapace-like skin and long tentacles. It still retains the basic shape and residual look of its host, but it is utterly alien. Also, a "male" Sil is much larger in this form than a "female".

Infection of a host enables an alien to breed with members of the host's species. Driven by an instinct to breed, these beings will continuously pursue this endeavor. The alien can sense genetic defects in beings and will not mate with them. It has been proven that Sil DNA cannot adapt to flawed DNA, and it will die if exposed to it. A Sil cannot change hosts, because it bonds genetically immediately upon entering a host creature.

No pureblood alien has yet been encountered. The ones on Earth have acted solely on instinct, not some grand racial scheme to destroy the Earth. They live to breed and survive and will destroy anything that tries to stop them. It is not known if there is a Sil society on some uncharted planet or if they merely act as a virus preying on everything they encounter.

A Sil is a very versatile creature capable of surviving in many extremes. In their natural form, a flowing pseudo-liquid with tentacles, they appear not to require air. They also seem to exhibit the ability to hibernate indefinitely until a suitable host is found. This is how they have survived on Mars when other life forms were unavailable. In natural form, this alien race cannot breed; however, it can grow at an amazing speed. There have been reports that one grews from the size of a single cell to the size of a small room in minutes. Its growth limits are unknown, because it was incinerated soon after. This species is required to bond with a host and use it to breed. A Sil is a true symbiote, and most of its advantages come from its ability to bond with any known animal. A Sil cannot use a plant as a host. Once a host is taken, the Sil acquires the instincts and thought processes of the being possessed as well as retaining its own. It can still be mildly influenced by things that normally influence the host, but the SIL is in control, so this is a dangerous endeavor.

If two Sils were to breed (requiring that they possess hosts fertile with one another), the resulting offspring would be a pureblood. Such a creature has never been encountered by man. A pureblood has no host, but it resembles the hybrid form of its parents. This is the only form this creature has. A pureblood is stronger and faster than its parents, and it is fertile, but only with other purebloods. Note that purebloods do not have to mate with other purebloods of the same host species.

A SIL that finds a host has some advantage over a Sil grown in a lab. The Sil automatically attains the knowledge of the host, and is aware of what it is. A Sil that is grown doesn't know exactly what it is and is forced to live by instinct. It also must learn about behavior. Fortunately, a Sil learns rapidly, but it still takes time. If properly isolated, a Sil grown with human DNA will not have knowledge of human behavior or language. It has been noted that a Sil grown with human DNA can be appealed to in human terms to a limited degree. This will not work in cases where instinct overrides such thoughts.