The viral life cycle starts when a viral particle invades a host cell. In plants, this could be through a lesion perhaps spread via an insect vector or by direct contact. In animals, viruses are transmitted by direct contact or by insects and gain entry into the host cell via phagocytosis. Viruses which attack bacteria are known as bacteriophages (‘phages) and enter into the cell by direct injection of the nucleic acid through the cell wall.

After gaining entry into a host cell, the virus has three options (Marsh and Wellington, 1994); either to become integrated into the hosts’ chromosome (lysogeny), to generate more viral particles and be released back into environment to infect other cells (lytic phase) or to exist halfway between a the two states (pseudolysogen) (Kimura et al, 2008). Lytic phages direct the hosts’ metabolism to produce more ‘phage particles which results in lysis of the host cell and hence its destruction releasing new virus into the environment ready in infect other cells. This strategy relies on sufficient host material to be available to sustain the production of the ‘phage inside the host. 

When the host population numbers are low, the virus may enter into a lysogenic phase in which the viral nucleic acid becomes incorporated into the hosts’ chromosome (the temperate phage). Here the virus may modify part of the hosts’ metabolism to produce for example changes in the hosts’ cell membrane. The viral DNA is reproduced each time the host cell multiplies. With a pseudolysogen, the infected host remains alive and multiplies whilst maintaining a lytic population of ‘phages.  Conditions where this may occur include where the host is starved of nutrients (for example in soil) and a balance is made for the phage between existing as a true lytic phages or as a lysogen. Viruses are unable to multiply in the absence of the host.

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