The parasites were black worm-like slugs which were capable of infesting other creatures. Once inside the body, they began a mutagenic change by forming a sac inside a host body. The creature emits radiation which appears to be part of its biology, and which begins to change the host's body into a much more monstrous, fleshy form. This results in the host becoming far more aggressive and violent as it begins to kill anything in its path. The change was quite evident, as even simple docile creatures became mindless brutes due to the presence of the parasite.
The creatures began plaguing the galaxy when Stargate Command began to deploy Sodan Invisibility devices for use by recon teams when scouting out Ori activity in the Milky Way galaxy. However, it was discovered that machines induced a radiation which was harmful to Humans but not Jaffa due to the latter's use of Goa'uld symbiotes. As such, the mechanism that produced the radiation was disabled allowing for use by SG teams.
However, it was learned that the radiation was a vital part of the process as it worked to repel the parasites before re-entry into the normal universe. Without this radiation, the parasites began appearing on the worlds where the device was being used. They began infesting various local animals and mutated them turning them into violent beasts that attacked various people.