Cultual contamination is the alteration of a culture's natural development by an outside influence or exposure to a more technologically advanced society. Cultural contamination can be sociological or technological and can have drastic consequences.
Cultural contamination was a concern among warp-capable species at least as early as the 20th century, by which time a Vulcan team stranded on Earth hesitated to interact with the humans. (ENT: "Carbon Creek") In the 22nd century the Vulcans had adopted a policy of non-interference with less technologically advanced societies. (ENT: "Broken Bow") Under the guidance of Vulcans, notably T'Pol, Earth's first warp 5 starship, the Enterprise slowly began to adopt a similar policy despite initial difficulties like those encountered on Valakis. (ENT: "Dear Doctor")
Repeated encounters of this type eventually lead to the creation of the Prime Directive for Starfleet which prohibits interfering with any pre-warp civilization as well as strict rules for initiating First Contact. (ENT: "Dear Doctor"; TNG: "Homeward"; VOY: "Caretaker", "Time and Again")
Even so, incidents of contamination are not rare and both accidental and intentional occurences have been recorded. Notable instances include the contamination of Sigma Iotia II by the Horizon, which lead to a complete alteration of the social structure of the planet, and the intentional interference on Ekos in an attempt to correct perceived flaws in the society. (TOS: "A Piece of the Action", "Patterns of Force")
Despite the dangers, the instigators of cultural contamination may attempt to repair the damage by revealing even more about themselves, or making the society further aware of the changes that have occurred. One example of this approach was Captain Jean-Luc Picard's resolution to the contamination of the society on Mintaka III when a Federation anthropological team was exposed there. (TNG: "Who Watches The Watchers")
Observer teams make use of specialized equipment such as duck blinds and isolation suits to avoid early First Contact. They may also undergo cosmetic surgery to allow themselves to better "blend in" with the populace they are observing. (TNG: "Who Watches The Watchers", "First Contact"; Star Trek: Insurrection)