POD PEOPLE / BODY SNATCHERS

The Pod People are a nomadic alien species of small gelatinous creatures originating from a dying planet. They evolved the ability to defy gravity and left their planet in search of a new world to settle on to float in space for millenia before occasionnally landing on suitable worlds. 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. They would then consume all of the planet's resources, only to then leave in search of yet another new world. It is hinted that such a consumption was the fate of the civilizations that once inhabited Mars and the Moon. 

Upon landing on a planet, they would try to replace the dominant species by  growing into large plant form which would spawn emotionless replicas from its pods; As a pod reaches full development, it assimilates the physical characteristics, memories, and personalities of any sleeping person placed near it; these duplicates are devoid of all human emotion. The assmiliation process would leave the original bodies to disintegrate into dust once the duplication process was completed. The duplicates had lifespans of only five years, and could not sexually reproduce.

Earth was among the planets the landed on. Stories of case where such cases happened

In 1956, they landed in a small California town. Alien plant spores grow into large seed pods, each one capable of reproducing internally a duplicate replacement copy of each human. A local doctor uncovers what is occurring and managed to stop the invasion with the help of the Federal Authorities.

In the 1978 invasion, the pod landed in San Francisco and managed to bring their effort further. The replicas showed the ability to communicate thru scream in a creepy alien voice to alert other pod people of those not yet taken and seemed to exhibit a kind of extra-sensory perception. Replicas merging more than one subject were also witnessed like a case of a merged man and his dog.

As similar scenario happened in 1993 in an Alabama Amry base.

In 2007, the invaders changed their method, from pods, they made contact with humans directly through liquids and then worked much like viruses which,  once the person had fallen asleep,  rewrote human DNA. The aliens then vomited 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".  However, it was discovered that people who had certain illnesses during childhood were completely immune to the virus. A vaccine was created and the entire pandemic is cured within a year; those infected, after being treated,we re unable to remember events during their infection, "as though they were in a deep sleep".  In one California town, though, the invaders used their traditional method, but with more egg-like pods.

Source: Jack Finney's  novel The Body Snatchers. (1954);

Invasion of the Body Snatchers (1956 film); Invasion of the Body Snatchers (1978 film); Body Snatchers (1993 film); The Invasion (2007); The invasion of the Pod People (2007)