There are more viruses on Earth than there are stars in the universe.Astonishing facts such as these pepper Carl Zimmer's A Planet of Viruses, a book whose ten short and clear chapters offer as good an introduction to these extraordinary entities -- microbiology's polymorphous Trimurti -- as you could wish for. 
If you stacked every virus end to end they would stretch 100,000 light years.
10 percent of all the photosynthesis on the Earth is carried out with virus genes.
Life as we know it may owe its existence to viruses.
Research into the nature of viruses and their significance to life as a whole has exploded in the last couple of decades. There is, surely, much more to discover. For now at least three fundamental points seem clear: viruses are ubiquitous, diverse, and have played a central role in the evolution of (virtually) all life forms.
|chart from Pennisi, Science, 25 March 2011
The genes in a human and the genes in a shark are quite similar -- so similar that scientists can find a related counterpart in the shark genome to most genes in the human genome. The genetic makeup of viruses, on the other hand, matches almost nothing. In a survey of viruses in the Arctic Ocean, the Gulf of Mexico, Bermuda and the northern Pacific, scientists identified 1.8 million viral genes. Only 10 percent of them showed any match to any gene from any microbe, animal, plant or other organism -- even [to] any other known virus. The 90 percent were entirely new to science. In 200 litres of seawater, scientists typically find 5,000 genetically distinct kinds of viruses. In a kilogram of marine sediment, there may be a million kinds.One speculation: if -- as has been suggested  -- the ancestors of viruses predate cells, could they have played a role in the emergence of life itself from what Martin Nowak has called 'pre-life'? , 
|A virus within this eukaryote, Cafeteria roenbergensis, has the largest genome of any known marine virus, with ~730,000 base pairs of double-stranded DNA. It may be a member a fourth domain of life.
 Like other reviewers such as Maggie Koerth-Baker at Boing Boing I received a free review copy.
 If I count correctly that's 1030 -- a nonillion.
 Eugene Koonin, Tatiana Senkevich and Valerian Dolja: The ancient Virus World and evolution of cells.
 'Life', suggests Nowak, can be seen as 'an infection of pre-life.' See Super-Cooperators by Martin Nowak and Roger Highfield, and Prevolutionary dynamics and the origin of evolution by Martin Nowak and Hisashi Ohtsuki.
 As Zimmer reports, Patrick Forterre has proposed that in an RNA world, viruses invented double stranded DNA as a way to protect their genes from attack. 'Eventually their hosts took over their DNA, which then took over the world. Life as we know it...may have needed viruses to get its start.'
P.S. In the second section of this essay drawing on his book The Mathematics of Life, Iain Stewart explores how the structure of viruses can be explained by maths.