Sometimes, individual data elements are not sensitive, but the linkages among them are. For instance, some person is named Erin, some person has the medical condition diabetes; neither of those facts is sensitive. The linkage that Erin has diabetes becomes sensitive.
Medical researchers want to study populations to determine incidence of diseases, common factors, trends, and patterns. To preserve privacy, researchers often deal with anonymized records: records from which identifying information has been removed. If those records can be reconnected to the identifying information, privacy suffers. If, for example, names have been removed from records but telephone numbers remain, a researcher can use a different database of telephone numbers to determine the patient, or at least the name assigned to the telephone. Removing enough information to prevent identification or re-identification is difficult and can also limit research possibilities.