Viruses can cause epidemics (infection of a population) such as seasonal influenza, or pandemics (infection spread across continents or even global) such as 1918 Spanish flu.

There continues to be discussion as to whether viruses are a form of life, protolife or just a parasitic organic structure. They share some characteristics of life forms – they possess genes, evolve and reproduce. Yet viruses do not possess many of the basic characteristics of living organisms, they do not possess their own cell structure or metabolism and cannot reproduce outside a host cell. The virus method of reproduction, spontaneous self-assembly, has more in common with crystal growth than with the cell division seen in living organisms. 

Viruses are sub-microscopic and most can only be seen with an electron microscope. They share basic characteristics and structure. All viruses have genes and a protective protein coat called a capsid. The structure of a virus is a particle or virion composed of nucleic acid (DNA or RNA), surrounded by a protective coat of protein called a capsid. The proteins that compose the capsid are coded by the virus’s genes and these provide the basis for its size and shape (morphology). Some viruses also have a lipid envelope composed of fat and protein molecules

Viruses must infect a host cell, because they rely on commandeering the cellular machinery of the host cell to synthesise proteins, grow and replicate.

The virus lifecycle depends on its ability to infect the host and consists of the following stages:

Attachment - binding to the host cell wall.

Penetration - entering the host cell.

Uncoating - degrading the virus protective coating to allow the release of the viral nucleic acid.

Replication - using the host cells mechanism to synthesise proteins and reproduce.

Assembly - producing multiple copies of itself.

Release - the virus uses a process called lysis to burst open the host cell wall, this kills the cell and releases the multiple copies of the virus.

The immune system of the host defends against virus attack, both in a general way through the innate immune systems – such as physical barriers, mucus, biochemicals, inflammation and fever responses. The host body also uses adaptive immunity, where antibodies and T cells identify specific viral fragments, then locate and eliminate the virus and infected cells. This process creates an immunological memory, which prompts a quicker stronger response if the virus is encountered again – this gives us immunity and is the basis of vaccination.

Although a virus reproduces versions of itself, it can produce a replica virus that is different from the original because of genetic change through reassorting, recombining and mutating genomes. This is why new strains of virus continue to emerge.

Viruses can infect all living organisms – bacteria, plants and animals, however most viruses have a limited host range, so in general they are restricted to related species eg common cold can infect humans and gorillas, parvovirus can infect dogs and foxes. Cross species viruses can spread across species, some of these viruses such as mad cow disease have been linked to modern, intensive animal husbandry – factory farming.