This is how a Varroa mite (aka Varroa destructor) looks like when your hive is infected with it. By clicking on the photo, it will enlarge.
The Varroa mite can only reproduce in a honey bee colony.
The mite enters a honey bee (mostly drone) brood cell and as soon as the cell is capped, the mite lays eggs on the larva. The young mites hatch in about the same time as the bee develops and leave the cell with the bee and spread to other bees and larvae.
The mites suck the hemolymph of the adult honey bees, leaving open wounds and then the bees are more prone to infections.