Tunguska Spore

The Spore is a seed of a creature that is in a Metallica's music video for their song "All Nightmare Long", that resembles an armored rat or worm. When in contact with inanimate matter, it produces another one of these creatures. However, when introduced to biological matter, the spores mimic the surrounding cells, becoming nerve, muscle, and skin tissue, much like stem cells.

History
The spore and its parent organism came to Earth in the Tunguska event, on June 30, 1908. In pictures taken after the cataclysmic detonation (likely taken by Leonid Kulik during his expedition to the impact site in 1921, which was the first expedition to the area following the disaster), a zombified deer was recorded. In 1938, Soviet research teams discovered a fine orange dust in the crater, which turned out to be the spore. The organisms were studied closely, where their regenerative properties were discovered. One patient with a serious laceration was swabbed with a spore solution, and in 8 hours, his wound healed completely; and a steak injected with the same solution began to behave like a cow. The next step was to test on putrefied tissue. A weeks-dead cat, named Lazarus by the research team, was injected with a spore culture, and began to move again. However, a disturbing trait was discovered: an increase in aggression in partially-decomposed organisms, with sporadic episodes of violence afterward, meaning they could attack anyone at any time.

Further research revealed that barium caused the death of Spore-derived tissues. With the organisms under Soviet control, the Spore was distributed over American airspace, causing all dead organisms to rise up and attack the living. Soviet intervention indebted the American people to the Soviet Union, and the USA was assimilated into the USSR. All undead creatures were decapitated, to make their recognition easier, and make them less dangerous. However one Zombie human escaped containment, thankfully decapitated...even so, it took over the research facility, and all survivors were evacuated.