A virus is a tiny infectious particle (nucleocapsid) surrounded by a protein membrane or a lipid envelope. The nucleocapsid contains a small amount of genetic information and any required replicating enzymes. The outer surface of a virus may contain proteins that recognize and bind to specific cellular receptors, which facilitates viral attachment and entry into cells. Once inside a host cell, a virus hijacks the normal cell processes to make many copies of itself and to produce viral proteins. Viruses are energy parasites; they do not generate their own adenosine triphosphate (ATP), so must derive all of their ATP from the host cell.
Viruses can be either double-stranded DNA or single-stranded RNA. They do not have mitochondria or other cell-like organelles, so are completely parasitic. The replication of a virus occurs as a burst of thousands of new viruses from one virus over a short period of time, producing tens of millions of virus particles per milliliter of culture media or blood. This is a very fast rate of replication compared to other organisms, and is the key reason that viruses are so infectious.
Viruses occupy a unique taxonomic position. They are neither plants nor animals, and cannot reproduce or carry out all of the other life functions that characterize living organisms. They are, in a sense, like bean seeds. A bean seed does not have the properties of life until it is planted in the ground and given the proper conditions. Similarly, a virus is not alive until it enters the body of a host organism and triggers certain events, such as cell lysis.