Xenocite

The Xenocites are alien "leeches" artificially manufactured by the Highbreed and are mass-produced and attached to a host, which create DNAliens.

Biology
As they attach to their host, they take over the personalities of the people they possess and control them (forcibly hacking their DNA). Within hours, the Xenocite will completely cover the host's bodies transforming him into a DNAlien.

Xenocites eggs are created from an egg-laying machine. The eggs hatch underwater in large artificial rivers (that look similar to the pools used to store radioactive material). After hatching, the Xenocites are stored in boxes filled with DNAlien slime, probably to keep them fresh.

Considering that they were seen living in open air and many were shoved into wooden crates with seemingly no DNAlien slime, it is implied that Xenocites are amphibious lifeforms.

Abilities
Xenocites merge with and change their hosts into DNAliens, taking the characteristics of the host species they are attached to and giving them standard DNAlien abilities. They can reprogram the mind of their hosts after merging with them.

Xenocites possess enhanced strength and agility that they use to avoid attacks and quickly latch on to a victim.

Weaknesses
Xenocites are unable to merge with silicon-based lifeforms, such as a Crystalsapien.

The Omnitrix's genetic repair function can separate a Xenocite from its host, thus negating the DNAlien transformation.

The DNA Repair Guns created by Cooper Daniels can destroy Xenocites, turning the hosts back into humans.