Macrovirus

The macrovirus is a Trekverse organism that injects or sprays host bodies with virus-laced slime. Infected individuals grow lethargic, pass out, and churn more bugs out of their necks until they die from dehydration. Voyager's EMH developed a cure.

The macrovirus begins life microscopically inside a host body. By absorbing growth hormones from infected hosts, it grows bigger until it emerges from the host into the macroscopic world. Usually, it is visible and audible at this stage. More detail can be found here.)

The aforementioned cure will stop the infection in a host body. An airborne macrovirus, sometimes called a bug, can be destroyed by an energy blast, killed by a breach in its outer shell (often caused by a pointy object or fast projectile), or by the cure. Of course, if it is too small for that, there is a problem.

The macrovirus has three observed instincts: to seek new hosts to infect, to consolidate existing hosts, and to struggle (slightly) if attacked.

The HQ Plague
The League of Mary Sue Factories started an epidemic in HQ on March 12 2008, HST, leading to a total quarantine. The initial epidemic, due to the unnatural size of an early host (Paul Bunyan), was known to feature specimens of the virus large enough to completely fill the height and width of an HQ hallway. All Agents were issued with TARDISes, and the town in New Caledonia was designated as a general meeting point. Tragically, during the attack, Makes-Things was killed, along with approximately one thousand other Agents.

Links
http://www.freewebs.com/agenttomato/mission5.htm

http://community.livejournal.com/tawaki_ppc/5882.html