**1. Introduction**

#### **1.1. Viruses: Special class of organisms**

Viruses form a major class of biological entities encompassing diverse environments ranging from algae in marine ecosystems to soil, plant, human and animal systems. Several metage‐

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nomic studies have revealed the possibility of viruses being the dominant species of our biosphere [1]. Deep sequencing efforts have shown that viruses form 106 –109 particles per millilitre of seawater [2]. It is also interesting to note that ~90% of the reads obtained from such experiments did not encode proteins, which are reported in other organisms, including viruses, that have been characterised so far. This clearly demonstrates that the actual viral diversity has not been sampled in an adequate manner so far. A crucial aspect of viral studies is the disease burden associated with them, which is known to be enormous with serious economic implications. World Health Organization documents that the global burden of communicable diseases (of which viral diseases form a major chunk) is ~15 million annually [3].

Beyond abundance aspects, study of viral evolution and genetic variations enabled the proposal of the virocentric standpoint of the evolution. Viruses gained centre stage for reasons such as being smallest replicating entities, having short generation time, large population sizes and high replication and mutation rates. Attributes such as variation in genome sizes, gene pool, shape and assembly of particles are responsible for viruses to attain pivotal role in the study of evolution [4]. It has been observed that all plausible replication and expression strategies have been employed by viruses to dynamically adapt to the ever-changing envi‐ ronments. Processes like complementation, recombination, reassortment, high mutation rate and existence as quasispecies enable the viruses to outgrow and outcompete the host immune system. The molecular forces driving these processes can be delineated by sequencing and the subsequent analyses.
