The concept of fruit of the poisonous tree is founded under the criminal law. The doctrine states that any evidence that is discovered due to information found through illegal searches or other unconstitutional means such as forced confession may not be introduced by the prosecution during trial. The theory flows from the analogy that if the tree, being the original illegal activity, is poisoned in the first place, then any fruit that grows from the poisonous tree is therefore tainted with the poison, being the resultant evidence that is derived from the illegal activity.
For example, as part of a coerced admission which was made without reading to a suspect the Miranda Rights (please kindly refer to my previous post "Point of Law - Miranda Rights"), the suspect tells the police the location of the stolen property. Assuming that the police does in fact finds the stolen property in the process, since the admission, being the poisonous tree, cannot be introduced as evidence during trial, the stolen property, being the fruit of the poisonous tree, can neither be introduced as evidence during the trial as well under the doctrine.
Generally, the simplest cases where the dotrine applies involves the exclusion of illegally obtained evidence. For example, an illegal body search of the suspect may yield the stolen property itself. The item, as in the above case the stolen property itself, which the defense is trying to suppress is direct or primary in their relationship to the illegal police action. Therefore, if the police have acted unlawfully, the evidence would then be excluded from trial.
However, in many occasions, the evidence is derivative or secondary in nature. For example, an illegal search may turn up a key to a public locker where the stolen property is being kept, or a coerced confession may reveal the place where a suspect hid the stolen property as mentioned in the above, or an illegal tap of the suspect's phone may reveal the whereabouts of a person willing to testify against the suspect. In such cases, it is necessary to determine whether the derivative evidence is fatally tainted by the initial or primary illegal action.