Earth is not the only planet to produce life. Several advanced alien races have visited earth, and have their own plans for what to do with humanity…
The Nordics (A.K.A Tall Whites or Pleiadians) appear completely human, their defining features being their pale skin and blond hair. Despite their human appearance, they have a number of unusual organs and entirely different biochemistry, causing most drugs intended for humans to have little, if any, effect.
The race known as Reptilians are perhaps the oldest race in the known galaxy. They are viscous and warlike, and tend to leave a trail of bodies wherever they go.
The race called the Greys have few if any known goals. They are most often found abducting humans or animals, and stories of experiments and hybridization are always on the mind of those who know.