Everybody does know that they are dangerous and inimical to human goals. Rumor of a Symbiot infestation is enough to bring a squad of Church inquisitors with flameguns, ready to burn first and ask questions later.
Symbiots are the result of an unusual chance encounter between a small band of (mostly human) eco-terrorists (among whom were a psychic and a theurgy-practicing Gjartin), and a dormant, ancient, parasitic, shapeshifting sentient race, the Xolotl.
The latter attempted to parasitize the band, but the occult sensitivities of two of the members allowed them to achieve/maintain conscious control over themselves and their bodies. Their newfound powers were numerous including a new catgory of occult power, involving awareness of the "lifeweb," various shapeshifting and life-shaping powers, and the ability to "infect" other living things with the same transformation they had themselves undergone.
Symbiots have conquered several planets since they begin their expansion from Chernobog. Their known Lifeweb is: