**Core Concept**
Mad cow disease, also known as Bovine Spongiform Encephalopathy (BSE), is a neurodegenerative disorder caused by a misfolded protein called a prion. Prions are infectious proteins that can induce normal proteins in the brain to also misfold, leading to cell death and brain damage.

**Why the Correct Answer is Right**
The correct answer is related to the transmission of prions between animals. Prions can be transmitted through the ingestion of contaminated food, such as meat from infected animals. In the case of mad cow disease, the prions are present in the brain and nervous tissue of infected cattle, and can be transmitted to humans who consume contaminated beef. The human version of mad cow disease is known as variant Creutzfeldt-Jakob disease (vCJD). The transmission of prions is a unique feature of this disease, as it does not involve a viral or bacterial infection.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because mad cow disease is not caused by a virus.
**Option B:** Incorrect because mad cow disease is not caused by a bacterial infection.
**Option C:** Incorrect because mad cow disease is not caused by a parasite.

**Clinical Pearl / High-Yield Fact**
It's essential to note that prion diseases, including mad cow disease, are resistant to heat, radiation, and many disinfectants, making them challenging to inactivate. This is why strict regulations have been implemented to prevent the transmission of prions through food and other means.

**Correct Answer: D. Prion transmission**