Viruses are ubiquitous. They are part of the ecosystem of earth and cannot be destroyed by any antibiotic treatment. Viruses are minimum of life form without any nucleus or cell wall. Though there are many kinds, in general virus is a strand of DNA or RNA surrounded by capsid whose shape is virus specific. The capsid is surrounded by one or more protein envelopes.
Viruses are thus a unique life form. The surface of viral protein is studded with receptors, specific kinds of sensory organs that tell viruses about their surroundings. Viruses use these elegant sensory organs to analyse the environment and to find the cells where they are most suited for their production. Viruses are very good at surviving. They can analyse the nature of immune response that occurs against them and they can alter themselves or alter the host immune defence itself.
DNA virus when it multiplies within the cell, it uses a biofeed back loop to make sure copies of itself are fairly accurate. In contrast, RNA virus tends to make copies which vary a lot from the original. Many atimes these copy differences are originated by RNA virus to increase their genetic variation and hence survivability and increased reproduction in the host cell.Efficacy of vaccine in core of DNA virus remains more or less stable and long lasting. In case of RNA virus it is very hard and sometimes impossible to develop efficient vaccine.
The body defence mechanism consists of proper immune system at a different level. The major components of immune system are thymus, spleen, lymph system, bone marrow, white blood cells, antibodies, complement system and hormones. In poultry bursa and ceca.
The immune system is designed to recognise the cells that make up the body and repel and kill foreign invaders including bacteria and viruses. White blood cells called macrophages destroy germs as soon as they detect them. However, if a viral infection begins to take hold, defence system uses more powerful 'B' and 'T' lymphocytes.