Mad cow disease is caused by __________.

Mad cow disease, scientifically known as Bovine Spongiform Encephalopathy (BSE), is a fatal neurodegenerative disease in cattle.

It is caused by prions, which are abnormally folded infectious proteins. Prions induce abnormal folding of specific normal cellular proteins found most abundantly in the brain, leading to brain damage and the characteristic signs of the disease.

Viroids are infectious RNA molecules that cause plant diseases, Mycoplasma are bacteria lacking a cell wall, and Aspergillus is a fungus.

Answer: prions