Viruses are microscopic particles of 20-300nm in size, obligate intracellular parasites; they can reproduce only by invading and taking over other cells as they lack the cellular machinery for self reproduction. Host range includes a wide variety of animal and plants. There are viruses which can infect bacteria and they are called batceriophage. A complete virus particle is called virion consisting of its simple nucleic acid (either RNA or DNA) and a protein coat called capsid. Viruses with DNA as genetic material are called DNA virus ( eg: herpes virus or Adenovirus etc) and those with RNA as genetic material are called RNA viruses ( e.g.: Foot and Mouth disease virus, SARS virus, HIV etc). Virus encoded proteins form the capsid. The nucleic acid and the capsid together termed as nucleocapsid. Certain viruses have a membrane or envelope covering the nucleoacpsid and they are called enveloped viruses. The envelope is usually of glycolipid and the lipid membrane itself and any carbohydrates present are entirely host-coded (The Influenza virus and HIV use this strategy to evade the immune system).
Since viruses use the machinery of a host cell to reproduce and reside within the host cell, they are difficult to eliminate without killing the host cell. Viruses used different mechanisms to evade the immune system of the host cell. The most commonly used strategies include antigenic variation, antigenic shift and drift. Influenza virus uses both antigenic shift and antigenic drift to evade the immune system. Antigenic variation can occur through three broadly defined genetic processes: gene mutation, recombination, and switching results in pathogens that are immunologically distinct from the parental strains there by escaping the immune system of the host .Antigenic shift involves between two different subtypes of the same virus coinfect a single cell and exchange RNA segments to form a new subtype. Antigenic drift is by changing the surface protein mainly haemagglutinin and Neuraminidase proteins in case of influenza virus. Mutations in the RNA or DNA causes change in the surface protein and host immune system fails to identify the viral proteins and this will facilitate the viral persistence in the host cell and making it difficult to control.
Reference:
Fileds Virology, David M.Knipe, Peter M Howley, Diane E Griffin, Robert A Lamb, Malcom A Martin, Bernard Roizman, Stephan E Stratus, Source: Lippincott Williams & Wilkins (LWW), 5th edn, 2007.