Viruses are the smallest living organisms, but without a cellular structure. They have a simple structure, consisting of a small piece of nucleic acid, either DNA or RNA, surrounded by a protein or lipoprotein coat. They can reproduce by invading living cells, therefore, are all parasitic. While outside a host cell they are inert, inside they enter a dynamic phase in which they replicate, pirating the host cell’s enzymes, nucleic and amino acids and machinery to accomplish what they are not equipped to do alone. Viral replication is often carried out at the expense of the host. Diseases such as herpes, rabies, influenza, some cancers, poliomyelitis and yellow fever are of viral origin. Of the estimated 1,000-1,500 types of viruses, approximately 250 cause diseases to humans (over 100 of which cause the common cold) and 100 infect animals.
Discovery of Viruses
It was in 1898 that Beijerink, a Dutchman, for the first time described the infectious nature of certain plant fluids as ‘virus’. Later, viruses were found to be the smallest living organisms, their size ranging from 20 to 300 nm, and about 50 times smaller than bacteria. They cannot be seen with a light microscope and pass through filters that retain bacteria.