How high must a fever be to cause hallucinations?

Depends: The degree of temperature elevation necessary to cause alterations in mental functioning will vary with the age and underlying neurological status of the person with fever. It is also difficult to determine (i.e., typhoid fever), whether it is just the fever or other aspects of the disease which produce hallucinosis. In either event, temps need to be higher than 104.5 to usually see changes.