The infestation bug is an insect on the planet Svoriin that presumably killed the entire population and took over the planet.

The infestation bugs origin homeworld is Skaald. A sting from the infestation insect can change a person's DNA so the inside of the body can grow many new infestation bugs. The huge insect species of BP6-3Q1 has a venom that will transform the DNA of the victim similarly to a virus, causing a metamorphosis into numerous new insects through equal matter conversion. Even Jaffa aren't immune to this venom.

A swarm of the bugs.

Each human would become approximately ten new insects, causing swarms in the millions within six to eight weeks. (SG1: "Bane")