Which of the following describes an infectious particle that causes damage to nerve cells in the brain?

The correct answer is prion. Prions are infectious agents composed entirely of protein, which lead to abnormal folding of proteins in the brain, especially in nerve cells. This misfolding causes neurodegenerative diseases, resulting in serious damage to the brain's structure and function. Prion diseases, such as Creutzfeldt-Jakob disease and mad cow disease, are infamous for their devastating effects on the nervous system, demonstrating how these infectious particles can lead to severe neurological disorders.

Virions, which are the complete virus particles, consist of nucleic acids enclosed in a protein coat and are known to infect host cells, but they primarily target various types of cells, not exclusively nerve cells. Bacteria, also distinct infectious agents, can cause a wide range of diseases but do not typically damage nerve cells in the same prion-induced manner. Lastly, while viruses can also affect nerve cells, they are not composed of protein alone and have a different mechanism of infection and pathology compared to prions.