The Lion's Head beacon is found by the Cylons as they pursued a probable course to Earth from the Lion's Head Nebula, as researched by their captive, Gaius Baltar.
The beacon is apparently an human artifact left by the Thirteenth Tribe as they made their way from Kobol to Earth.
When the beacon is brought aboard the basestar, it soon infects the Cylons, including agents, Raiders, the Hybrid and the basestar itself. The Cylon Centurions, apparently controlled by the Hybrid, began shutting down as the infection progressed.
Baltar's basestar crew immediately quarantines their Resurrection Ship to prevent the reception of any transmitted consciousnesses from the dying basestar. They discovered quickly as the infection could spread into their community via their ressurrection process.
Baltar is dispatched to the basestar to investigate. The beacon, as seen by Baltar, appeared to have a gelataneous material within slats in the interior.