The viruses are about one-hundredth of the size of the bacteria. They are obligate parasites. Viruses were studied prior to the development of bacterial knowledge. According to the definitions viruses are sub-micro topical entities capable of being introduced into the specific living cells and reproducing inside such cells only. They are made up of three distinct layers. The genetic material of the virus can be either DNA or RNA, they carry the genetic information, the viral proteins that are presently covering these genetic materials and protecting them and a cover of the lipid molecules forming a capsule. The study of viral replication and propagation is very important for the analysis of viral pathogenesis. As a result of this infection, the viral disease occurs in the body and they are able to multiply inside the human body. The symptoms of this viral disease vary from coughing to severe stroke. When a virus enters the human body, it gets itself attached to the host cells mechanism and they replicate inside them producing the viral genes and in turn the viral proteins. (Wagner and Hewlett 2004).These Viruses are quite different from another type of cells. They will contain either DNA or RNA and never both together. They depend upon the host to which they get attached for energy and raw materials that are required for viral replication. They are of various shapes and sizes ranging from 20 nanometers to 0.3 micrometers and the shape may vary from bullet-shaped, helical, polygonal, spherical, pyramidal to very complex structures. These viruses are classified based on the similarity of the viral components, their size of the nucleic acid. Based on this category Baltimore has classified the viruses into several types such as dsDNA Viruses, ssDNA viruses, dsRNA Viruses, +ve RNA Viruses, – ve RNA Viruses, RNA reverse transcribing viruses and DNA reverse transcribing viruses.