Which of the following describes a disease-causing particle characterized by no nucleus, a protein coat, and an RNA or DNA core?

A disease-causing particle characterized by no nucleus, a protein coat, and an RNA or DNA core is best described as a virus. Viruses are unique infectious agents that are distinct from other microorganisms such as bacteria, protozoa, and fungi.

The defining feature of viruses is their structure, which consists of genetic material (either DNA or RNA) encased in a protein coat known as a capsid. This simple structure allows viruses to hijack host cells to replicate and spread. Unlike bacteria, which are single-celled organisms that can live independently and possess complex cellular structures, viruses cannot replicate without a host. Additionally, unlike protozoa and fungi, which are typically larger and contain a nucleus along with cellular organelles, viruses lack cellular components altogether.

This unique combination of characteristics makes viruses distinct from the other options, leading to their classification as non-cellular life forms that can cause various diseases in humans, animals, and plants.