Do adults with fever get hallucinations or delusions?

They can: Many febrile illnesses, especially in the elderly can cause delirium, of which delusions and hallucinations, especially visual, are symptoms.

Yes: Can have vivid hallucinations.

They can: Particularly if the fever is quite high or the person with the fever has a coexisting compromise of the brain like any of the forms of dementia. Remember though, it's not that common for healthy adults with a fever to hallucinate and may suggest a primary infection involving the brain or connective tissue around the brain.