VIRUS
A virus is a very small particle that is capable of infecting a cell and potentially causing disease. The cell that is infected by a virus is called the "host cell". Most viruses are too small to be seen with a normal microscope but can be visualized using an electron microscope. Although there is some debate whether viruses should be classified as living organisms or not, generally viruses are considered as microorganisms. The study of viruses is called virology and is is a branch of microbiology.
Because a virus is unable to reproduce without the help of the host cell, by a strict definition a virus cannot therefore be considered as a living organism. One of the required chapacteristics for a living organism being the capacity to reproduce. A virus is, however, capable of reproducing within the host cell by making use of the cellular processes of the host cell. Virtually all cells are susceptible to infection by viruses. Different types of viruses infect different host cells. Thus there are viruses that infect plants, others that infect bacterial cells (these viruses are called bacteriophage or phage), and viruses that infect humans and other mammals. Both eukaryotes and prokaryotes can be infected by viruses.
All viruses contain genetic material, either DNA or RNA, enclosed in a case made of protein. Viruses vary greatly in shape and size. Not all viruses carry disease. Antibiotics have no effect on viruses. Some anti-viral drugs are available that can be used to treat viral infections.
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