A microorganism capable of infecting almost all plants and animals, including bacteria, and so small it can only be seen under a powerful microscope is a __________.

• A. Virus
• B. Bacteria
• C. Fungus
• D. Protozoa

Answer: (A)

Viruses are ultramicroscopic, non-cellular particles that cannot reproduce or carry out metabolism on their own; they must hijack a host cell’s machinery to make copies of themselves. Because of this dependency, they’re seen only with powerful microscopes like an electron microscope, and many viruses can infect a wide range of hosts, including plants, animals, and even bacteria (bacteriophages). The other options are cellular organisms that can be seen with light microscopes and do not have the same universal host range or the same requirement to hijack a host cell to replicate. This combination of extreme small size, reliance on a host, and broad host infection profile points to a virus.