Black Gears

What is a Black Gear?
In the television animé, Digimon Digital Monsters; A Black Gear is an item, created by Devimon to take control of good digimon. It looks like a gear in appearance, and in order to turn a good digimon bad, it attaches itself to the host, taking over thier body.

The Infected.
Digimon who were infected by Black Gears include:

-Meramon -Centarumon -Frigimon -Mojyamon -Monzaemon -Leomon -Whamon -Drimogemon

Once the Black Gears infected these digimon, they instantly realized that the Digidestined needed to be destroyed. The last digimon to be infected by a Black Gear was Drimogemon. The last Black Gear was seen in the episode Departure for a New Continent.

After Drimogemon's Black Gear was destroyed, the Black Gears were never seen again on the show.

The More, the Merrier.
In the digimon episode The Legend of the Digidestined, Devimon prepares to fight the Digidestined. However, before he does so, he speaks with Ogremon and a few seconds later, Ogremon is transformed into several Black Gears. He then combines with Devimon giving him the ability to grow.

The technique which Devimon used with Ogremon is similar, to that which he done to Leomon earlier on in this episode. While the digimon were fighting dark Leomon, several more dark gears entered Leomon, making him grow bigger, stronger and darker.