Aliens (The Puppet Masters)

A race of parasitic aliens act as the central antagonists of the 1994 sci-fi film, The Puppet Masters, which is based on a 1951 novel of the same name.

Overview
These mysterious slug-like aliens arrived on Earth via a UFO and began to infest the human population via latching onto a victim's back and taking control of their nervous sytems, turning them into slaves (hence the titular nick-name of "Puppet Masters").

A trio of government agents investigate these events and battle against the alien invasion, which becomes critical as the parasites spread rapidly and begin to infect military sent to stop them as well as numerous civilians.

It is discovered that the aliens share a hive-mind and are planning the complete dominantion of the human race, yet they are weak to electricity - which frees some of their victims and further investigation also leads the heroes to find some individuals whom the aliens can not possess.

This would play a part in defeating the aliens for good as biological warfare is applied, killing off the aliens - though it is unknown if more of these creatures exist off-planet.

Trivia

 * the film's tagline of "Trust No One" is also the tagline of the X-Files, an unrelated sci-fi series (but very popular at the time).