viruses are the simplest entity in the firm of unique particles. he word virus is derived from the latin word "venome " which means "poisonous fluid". they are so small that they cannot be seen with light microscope. They can be studied under electron microscope or presence can be confirmed through biochemical tests. They are 10 to 1000 times smaller than bacteria. Like living organisms they produce themselves. They do not respire or excrete and can be crystallized.
viruses are inactive outside the living host cells and show life functions only inside host cells. Therefore, viruses are the only organisms in nature to show both non-living and living charactaristics. Whereas normal cells cannot be crystallized, and crystallized cells cannot be revived, viruses can do both. Viruses are nucleoprotein structures. They consist of protein envelope containing a nucleic acid (DNA or RNA ). They have no cytoplasm or metabolism. They use the metabolism of the cells in which they live.
Studies with the electron microscope show that viruses may be quite variable in size. Being very small (10-40 nm), viruses are placed at the bottom of the organizations of organisms.Information about viruses is obtained from studies of phages and bacteriophages (viruses that feed on bacteria). A bacteriophage virus has DNA inside and a complex protein envelop ide called the "capsid". The viral envelope of the tobbacco mosaic virus has been shown to include 158 amino acids and 16 types of proteins.
Each type of virus infects certain cells. yellow fever virus infects liver cells. Viruses are not affected by antibiotics but effected by only some physical and chemical factors like pH, high temperature and dehydration.