**2. Role in pathogenesis**

**Human viruses**: More than 200 viruses are known to infect humans and number is increasing with time, but the diversity of viruses suggests a huge number of viruses still unknown. In humans, yellow fever virus was the first pathogenic virus discovered in 1901. 1900 was the era of human virus discovery and most of the common pathogenic viruses studied during this time. In current scenario, two out of three infection causing organisms are viruses [4] and known to cause a variety of disease ranging from normal acute infections such as common cold, flu, and gastroenteritis to deadly diseases such as Hantavirus pulmonary syndrome (Huntavirus), AIDS (HIV) ebolavirus disease (ebolavirus). Recent outbreaks of viruses show the emergence of previously known viruses with modified virulence properties.
