Is it possible to develop dementia by eating bone marrow? A psychiatrist once told me to avoid eating bone marrow because it could contain prions that could cause dementia. However, I haven't been able to find any studies that confirm this. Could this happen?
You might be referring to Creutzfeldt-Jakob disease (CJD). It is transmitted through exposure to brain tissue and spinal cord fluid from infected individuals. 


  CJD is characterized by rapidly progressive dementia. 


The leading scientific theory at this time maintains that CJD and the other transmissible spongiform encephalopathies (TSEs) are caused by a type of protein called a prion.

CJD is indeed caused by prions: proteins that occur in both a normal form, which is a harmless protein found in the body’s cells, and in an infectious form, which causes disease. The harmless and infectious forms of the prion protein have the same sequence of amino acids, but the infectious form of the protein takes a different folded shape than the normal protein. Sporadic CJD may develop because some of a person’s normal prions spontaneously change into the infectious form of the protein and then alter the prions in other cells in a chain reaction. 

Sources
- NIH Fact Sheet
- WHO