The alien creature is a massive, plant-like lifeform which dwells beneath the surface of the planet. It can grow at an alarming rate, visible to the naked eye, and can spread on both organic and inorganic material. It is highly resistant to harm. Destiny's entire supply of chemicals, including squiggler venom and hydrochloric acid, had no effect. It can also withstand large amounts of heat; a small sample required a sustained burst from a flamethrower to kill. It is sensitive to the energy burst given off by a Stargate during its activation, but this only drives it back momentarily.
The alien creature spreads in two ways. It's primary method of spreading is to consume any available material around it, converting it at a rate that "smacks right up against the laws of thermodynamics," according to Lisa Park. When it senses another lifeform, apparently through vibrations in the ground, it springs forth tendrils to sting them, infecting that lifeform with microscopic spores that rapidly spread throughout the host body. The host survives long enough to spread the spores even further.
In 2009, the creature infected Matthew Scott when his team arrived on its planet. It continued to follow the team, attempting to infect the rest of them. Chloe Armstrong, believing she might be immune thanks to her exposure to the Nakai pathogen, deliberately allowed one of the tendrils to sting her to test her theory. Confirming that she was immune, Tamara Johansen then gave Scott a transfusion of Chloe's blood, allowing his system to fight off the infection. (SGU: "Cloverdale")