The microscopic organisms of P5C-353 created an orb in which they could survive after their world had died. There they slept for 100,000 years until the orb was brought back to Earth by SG-1. Once exposed to the atmosphere of a living world, the orb activated and impaled O'Neill in the gateroom. From there, the organisms went forth and multiplied. So it was written on the orb.
The organisms are mobile like bacteria, but small like a virus, and they bond in chains. They are slightly radioactive, and can become visible under ultraviolet light. They infested O'Neill and SGC personnel, causing an infection and high fever which responded temporarily to antibiotics, and they infested the base computers, initiating the autodestruct sequence. The organisms absorb energy, and attempts to use weapons against them only increased their strength and number. They reproduce exponentially, they require oxygen but can go dormant in an anaerobic environment, and they metabolize virtually any substance as if it were living tissue, similar to necrotizing fasceitus, or flesh-eating disease.
The organisms are intelligent and capable of communicating through others once reaching critical mass, and they refer to themselves only as "We." Through O'Neill, the organisms were able to communicate, and they agreed to be sent to P4G-881, a primordial world with no civilization, where they could live and multiply.
Episode Reference: Message in a Bottle