Contamination
Contamination was the introduction of a foreign substance or concept to a formerly pure or unaltered substance, either intentional or accidental.

In 2151, Doctor Phlox found the water supply of the Novans contaminated by the irradiated debris from the asteroid impact of 2083. This caused the sickness of the Novans such as Nadet's lung cancer. ()

A contaminated drink was said to be spiked. In 2154,  compared the residual effects of Orion pheromones on the crew of  to someone having spiked the water supply. ()

In 2266, Spock noted that spectro-readings of Psi 2000 showed no contamination or unusual elements. Montgomery Scott noted that there might still be some present that were not detectable by their tricorders, which Spock agreed to. ()

In 2366, the water supply on Beta Agni II was contaminated with tricyanate,
necessitating the introduction of hytritium in order to neutralize it. ()

See also 
 Contamination protocol
 Cultural contamination
 Foreign substance

External link 
 
Science