A secret agreement or cooperation between two or more people who are trying to deceive.
In health care, collusion implies any information (about the diagnosis, prognosis, and medical details about the person who is ill) being withheld or not shared among individuals involved.
Collusion also means that relevant and complete medical information is selectively or not disclosed at all to patients and/or relatives.

Patients have a right to an honest and full explanation of their situation and should be told as much or as little as they want to know; they also have a right to decline information.

Problems arising from collusion:
• Patient is unable to make informed decisions about treatment. Patient autonomy is compromised.
• Patient is unable to plan his or her affairs accordingly.
• Patient may guess the diagnosis/prognosis from other clues. This can lead to anger and breakdown of trust toward caregivers.

Chaturvedi SK. Communication with relatives and collusion in Palliative Care: a cross-cultural perspective. Indian Journal of Palliative Care, 2009, 15:1, pp. 2-9.