Alien seeds hitch a ride to earth in a space shuttle crew and begin to grow. When their numbers reach the Threshold amount they will be an unstoppable swarm.
1984
Doctor Who: The Sontaran Experiment
Predator
War of the Worlds
Mars Attacks!
Pixels
Signs
Doctor Who: Terror of the Zygons
Doctor Who: The Dalek Invasion of Earth
Attack of the Giant Teacher