A virus is an infectious microbe consisting of a segment of nucleic acid (either DNA or RNA) surrounded by a protein coat. It cannot replicate alone and must infect cells and use components of the host cell to make copies of itself. Viruses are not considered living organisms because they cannot perform metabolic processes without a host cell. They are found in almost every ecosystem on Earth and are the most numerous type of biological entity, infecting all life forms, from animals and plants to microorganisms, including bacteria and archaea.
Viruses are known to cause a wide range of diseases in humans, such as AIDS, COVID-19, measles, and smallpox. They can also infect other organisms, including fungi, bacteria, and plants. Viruses are quintessential parasites, depending on the host cell for almost all of their life-sustaining functions. They are not considered to be organisms and are generally placed in their own kingdom due to their unique characteristics and inability to reproduce and carry on metabolic processes without a host cell.
