Infected masks were Kanohi masks tainted by Makuta's darkness. They were created when Kraata made physical contact with a mask. They could also be infected at long range by a shadow Kraata. An infected mask took on a rusted, pitted appearance. Anyone wearing such a mask became subservient to the will of Makuta.
Infected Kanohi masks were tainted by the evil power of Makuta. They appeared scarred, rusted, and pitted. Anyone who donned an infected mask became a servant of Makuta and would obey his will until the mask was removed.
The Brotherhood of Makuta apparently made use of Rahi controlled by infected masks in their war with the Dark Hunters.
The infection of a mask essentially formed a connection between the being wearing it and a Makuta. The Makuta did not need to actively focus on each and every being wearing one of his infected masks to control them. A Makuta could not infect another Makuta's mask.
If an infected mask and a krana were placed on the same Rahi, the influence of the infected mask would win out, since Makuta's essence was much more powerful.