Final answer:
A virus is a submicroscopic particle that infects host cells and reproduces by using the host's mechanisms. It consists of genetic material wrapped in a protein coat and cannot reproduce or carry out life processes on its own.
Step-by-step explanation:
A submicroscopic particle that infects and resides in the cells of a biological organism is a virus. Viruses are acellular microorganisms, meaning they are not composed of cells themselves. They consist of a core of genetic material, either DNA or RNA, and a protein coat called a capsid. This structure allows viruses to infect a host cell and replicate by co-opting the host's cellular mechanisms. Unlike bacteria, fungi, or protozoans, viruses are much smaller, usually ranging from about 20-300 nanometers (nm), and cannot be seen under a light microscope but are visible using an electron microscope. They are considered obligate intracellular parasites because they can only reproduce within a host cell.