"The Immunity Syndrome" is a second season episode of the original science fiction television series Star Trek, first broadcast on January 19, 1968, and repeated June 7, 1968. It is episode #47, production #48, written by Robert Sabaroff and directed by Joseph Pevney.

In this episode, the crew of the Enterprise encounters an energy-draining space-dwelling organism.

The Federation starship USS Enterprise is en route to Starbase 6 for rest and relaxation. The communications officer, Lt. Uhura, receives a garbled distress call, but all she can make out are sector coordinates and the name of the ship, the USS Intrepid, manned entirely by Vulcans. Suddenly, the signal is gone and First Officer Spock, himself half-Vulcan, shudders and feels uneasy. When asked what is wrong, he replies that the Intrepid has "died".

Starfleet sends a priority message to Captain Kirk and orders him to investigate Sector 39J where contact has been lost with the colonies in the Gamma 7A system, also the last known position of the Intrepid. Arriving there, Ensign Chekov reports that the sensors show no life readings in the system of a billion inhabitants.

Meanwhile, Spock is checked out by Chief Medical Officer Dr. McCoy in sickbay where he explains he felt the combined shock and terror in the minds of 400 of his fellow Vulcans aboard the Intrepid as they died. McCoy is amazed that Spock felt anything with the great distance involved between the two ships, but admits there is a lot about Vulcans he still doesn't understand.

Spock returns to the bridge just as Uhura announces she has lost contact with Starfleet. Kirk has Spock scan a dark ominous form that appears on the main view screen. Spock reports it is some kind of energy turbulence; probably responsible for the death of the system inhabitants and the Intrepid crew. Kirk launches a sensor probe into the void. Suddenly a painful, high pitched noise fills the ship that renders half the crew sickened or faint.

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