Droon

The Droon were a species of parasitical insects, resembling a cross between an insect and a walking razorblade.

In their larval state, they preferred to live in the nasal passages of humans and are normally harmless, causing the host minor cold-like symptoms but no other problems. Prior to reaching their adult stage, they would leave their hosts when they sneezed. Normally, the host would remain completely ignorant of their existence, but sometimes the larva would pupate. In such cases, Torchwood would intervene to remove the larva. (TWN: Border Princes)