Pod People

The Pod People are an unnamed alien race that appears in the 1956 movie Invasion of the Body Snatchers and its numerous remakes. Although their backstories and forms vary between movies, it is generally assumed that they are fleeing their dying homeworld and seek to control and dominate Earth and render the native Humans extinct.

History from the novel
The Pod People are a race of nomadic, extraterrestrial parasites originating from a now dying planet. Realizing that it was only a matter of time before the planet's resources would be completely depleted, the pods somehow evolved the ability to defy gravity and leave their planet's atmosphere in the search of a new world to colonize. For millennia, the pods floated in space like spores, propelled by the solar winds, some occasionally landing on inhabited planets. Once there, they would replace the dominant species by spawning emotionless replicas; the original bodies would then disintegrate into dust once the duplication process was completed. They would then consume all of the planet's resources, only to then leave in search of yet another new world. Such a consumption was apparently the fate of the civilizations that once inhabited Mars and the Moon. The Pods' sole purpose in life was that of individual survival, with no attention given to the civilizations they conquered or the resources they squandered. The duplicates had lifespans of only five years, and could not sexually reproduce. Their invasion of Earth was short-lived: unable to tolerate the sheer determination humanity displayed in defense, the Pods abandoned the planet, leaving behind a small population of duplicates, who died shortly after.

Invasion of the Body Snatchers (1956 film)
One of the pod people hints at their extraterrestrial origin and purpose without fully explaining it. In the end the hero, played by Kevin McCarthy, gets away from the town and tells his story to a psychiatrist. A truck carrying pods is involved in an accident; thereafter the psychiatrist believes the story. He then asks the FBI and police to quarantine the town. The audience is left to only wonder whether they were successful or not. The original ending was less hopeful about the fate of humanity, ending before McCarthy escapes to a psychiatrist. The final shot is of him standing in the middle of a highway shouting warnings at passing cars and then directly at the camera.

Invasion of the Body Snatchers (1978 film)
The origin remains the same, though again only suggested. In this film, we see the aliens in their pre-invasion form. They appear as gelatinous creatures who abandon their dying world somewhere in deep space. Once they land on Earth, they assimilate leaves and become pink flowers; the aliens eventually begin growing the larger, six-foot long pods. This time, those subverted can scream in a creepy alien voice, used to alert other pod people of those not yet taken. They also seem to exhibit a kind of extra-sensory perception. When one character stabs his almost formed pod double, another pod person immediately emits the alien scream.

This version does not end with the same hope with which the novel and previous movie do, but ends with the pod people taking over almost everyone on Earth. The movie shows several ships being stockpiled with pods to be sent out into other parts of the world. In the very last scene, Veronica Cartwright's character is happy to see the hero, played by Donald Sutherland, only to hear him emit the alien scream. It is thereby implied that despite their best efforts, they were unable to stop the alien force.

There is a difference in the pods between the original and the remake. In the original, the pods burst open and begin duplicating Miles and his friends while they are wide awake. In the remake, the pods and flowers stay dormant until the humans are asleep.

Body Snatchers (1993 film)
These pod people emit the same scream to indicate non-converts. Their extraterrestrial origin is hinted at even less (only suggested through a pan-in of the galaxy during the opening credits). These pods also shrivel and disintegrate when they are killed, just like the bodies of the originals. As with the 1978 remake, this version seems to preclude any hopeful conclusion by the ambiguous ending in which the two leads land after seemingly destroying the pod people, only to find that the pilot who helped them land has been converted by the aliens, reflecting back to the eerie warning earlier stated: "There's no one like you anywhere."

The Invasion
In The Invasion, the invaders are changed from pods to an alien virus that is contracted through liquids. Once the person falls asleep, the virus rewrites human DNA. The aliens then vomit a gelatinous substance into liquids to help the invasion continue. When the invasion gains considerable strength, the pod people transform humans by directly injecting them with the substance, under the guise of "influenza vaccines". As it continues across the globe, entire world conflicts are resolved, including the Iraq War and Darfur. However, it is discovered that people who had certain illnesses during childhood were completely immune to the virus. A vaccine is created and the entire pandemic is cured within a year; those infected, after being treated, are unable to remember events during their infection, "as though they were in a deep sleep". Similar to previous incarnations, the virus can kill its human host. This is hinted at when Carol takes a picture of a human being converted and sends him into cardiac arrest. It is actually evidenced during Carol and Gene's exchange on the commuter train:

Carol: Where are your parents Gene? Gene: They didn't survive. Your family is my family now.

Invasion of the Pod People
In the 2007 film by The Asylum - which was a mockbuster released alongside The Invasion - the aliens designate themselves as "Pod People", and hatch from large, egg-like forms rather than organic seed-pods. Like the original Invasion of the Body Snatchers, they arrive in an isolated town and take the form of the townsfolk, whilst at the same time delivering more pods to other nearby towns and cities to aid their colonisation of the Earth.