Introduction to viruses
Viruses are non-living biological agents which can infect cells.
A virus is essentially genetic material enclosed in a protein coat.
Viruses are non-living because they cannot reproduce or perform metabolic processes.
Viruses require a host cell to complete their reproductive cycle. They are therefore classed as obligate parasites, meaning that they are 'obliged' to be parasites.
Viruses possess some features characteristic of living organisms. For example, viruses carry hereditary material that is passed through generations. This means they can evolve in response to external stimuli.
Infection occurs when a virus enters the host cell.
All viruses need to be inside a host cell in order to replicate, so all viruses are infectious.
The term infection is often used interchangeably to mean disease. However, they are not the same thing and should not be confused.
Disease refers to the health of the organism, so if the virus impairs the function of the organism, it is disease causing.
Disease results in symptoms, such as fever and pain, whereas infections can be asymptomatic (no symptoms) and do not necessarily affect the health of the organism.
All viruses are infectious, but not all viruses result in disease.