It has been estimated that every year two new species of human virus are found. According to researchers, 10-40 new species may be discovered upto the year 2020.
Less than 200 (almost 188) of about 1400 disease causing species were found to be viruses upto the year 2008.
It has been found that since last 50 years virus discovery per year has not been changed dramatically.
A large number of new human viruses came from the animal sources.
References and Further Reading:
Woolhouse, M. E. J. et. al. (2008). Temporal trends in the discovery of human viruses. Proceedings of the Royal Society, 275(1647), 2111-2115.