Ancient probe launched from the planet Iconia whose software overwrote the computers of the USS Yamato, destroying it, and then infected the Enterprise-D and a Romulan warbird.
The Iconian probe was a device launched from the surface of Iconia, homeworld of an ancient civilisation that had been destroyed some two hundred thousand years earlier. It carried a powerful software program that rewrote the computer systems of any ship it scanned.
In Contagion, the Galaxy-class USS Yamato was scanned by a probe while investigating Iconia in the Romulan Neutral Zone, and its systems failed until the ship exploded. The Enterprise-D downloaded the Yamato's logs before the explosion and became infected in turn, as did the Romulan warbird Haakona.
When a second probe was launched as the Enterprise reached orbit, La Forge realised the probes were the source of the contamination and had it destroyed. Picard, Data and Worf beamed to the planet, where Data was also infected before recovering, and the crew restored their ship by shutting down its computer and reloading it from protected archives. Picard then destroyed the Iconian launch facility.