ABSTRACT

Viruses are submicroscopic infectious agents that range in size from about 20 to 400 nm in diameter (1 nm = 10−9 m), the majority of which are small enough to pass through conventional sterilizing fi lters (0.2 μm). Each viral particle, or virion, consists of a single-or double-stranded nucleic acid

(either deoxyribonucleic acid [DNA] or ribonucleic acid [RNA] ) and at least one protein surrounded by a protein shell (known as capsid), whose shape varies from simple helical and icosahedral (polyhedral or near-spherical) forms to more complex structures with tails or an envelope. In addition, some viruses may possess an outer envelope composed of lipids and proteins. The protein capsid provides protection for the nucleic acid while other proteins (enzymes) enable the virus to enter its appropriate host cell. Viruses are classifi ed on the basis of their nucleic acid content, their size, the shape of the capsid, and the presence of a lipoprotein envelope, and they are divided into two primary classes: RNA viruses and DNA viruses.