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It is possible for viruses to live in mutualistic relationships with their hosts, these associations are often overlooked due to the devastating effect that many viruses can have.

To give an example in humans, when HIV-1-infected patients are also infected with hepatitis G virus, progression to AIDS is slowed significantly (Heringlake et al., 1998; Tillmann et al., 2001). Also hepatitis A infection can surpress hepatitis C infection (Deterding et al., 2006).

There are many other notable examples within plants, fungi, insects, and other animals, reviewed by Shen (2009), and Roossinck (2011), in two excellent papers.

The table below, summarises some beneficial viruses across all organisms, and is taken from Roossinck (2011).

References