Polycocyx astris

From Memory Alpha, the free Star Trek reference

Polycocyx astris is a form of space dwelling microbe that resides in interstellar dust.

While answering questions sent from Miss Malvin's fourth grade class at Worley Elementary School in Kenmare, County Kerry, Ireland, Doctor Phlox recalled the Polycocyx astris as a particularly virulent species. They have been known to lie dormant for millions of years before infecting a host and causing a 'nasty cold'. (ENT: "Breaking the Ice")