Data worm is an upgrade that gives the Hijacker the ability to infect enemy units with a virus that deals damage to them over time, eventually destroying the target. Upon destruction, the virus can transfer to another unit.
This upgrade also applies to the Heroic Hijacker summoned by the Hand of Legion power. A viable tactic is to infect a weaker unit, then hijack a stronger one nearby. If the weaker unit is even still alive once the stronger unit is hijacked, it will quickly fall to the stronger unit.