What is an isolated but infectious virus particle found outside the host cell known as?

A virus particle that is isolated and infectious when found outside of a host cell is referred to as a virion. A virion is the complete, extracellular form of a virus that is capable of infecting a host cell. It typically consists of genetic material, either DNA or RNA, surrounded by a protein coat known as a capsid, and in some cases, an outer lipid envelope.

Understanding the structure and function of a virion is critical in virology, as it represents the state in which viruses are transmitted from one host to another. This makes it fundamental in the study of viral infections and the development of vaccines and antiviral therapies.

Other terms in the context of virology serve different purposes: a bacteriophage specifically refers to viruses that infect bacteria, while the term pathogen encompasses a broader category that includes any organism that can cause disease, such as bacteria and fungi, in addition to viruses. Prions are infectious agents composed solely of proteins that can cause neurodegenerative diseases but are distinct from viruses, which contain nucleic acids. Thus, the correct identification of a virion is essential for understanding viral biology and transmission.