Viruses are distinct biological entities with the following properties:
1. A virus is an infectious, obligate intracellular parasite.
2. The genetic material of a virus is either DNA or RNA.
3. The genetic material of a virus enters a host cell and directs the production of the building blocks of new virus particles (called virions).
4. New virions are made in the host cell by assembly of these building blocks.
5. The new virions produced in a host cell then transport the viral genetic material to another host cell or organism to carry out another round of infection.
Viruses are easy to understand when we reduce their properties to simple descriptions such as those listed above. The confounding issues lie in the details – and with viruses, there are many, many details.