Viruses; Cells or not?
Viruses do not have a membrane as all other cells do, and rely on host cells to survive. Viruses can be thought of as sub-cellular parasites which infect living cells and then take over the cell's metabolic processes to produce more viruses. Viruses are not cells but they contain substances characteristically found only in living cells.
A virus is not a living organism. It is simply a protein box which holds a tiny strand of nucleic acid - a substance found in the nuclei of living cells. Viruses come in various shapes and sizes. Viruses are so small. A virus can only be visible with an electron microscope. Of all the characteristics of living things, movement, feeding, respiration, excretion, growth, sensitivity and reproduction, only reproduction is relevant. Viruses do not reproduce in the sense that cells do because new viruses do not arise from pre-existing viruses. Outside of the host, viruses are inert and inactive and viruses can be isolated and crystallized almost like a chemical compound. Only in a genetic sense can one think of a virus as a living thing, since a virus can certainly control the genetic make-up of the next 'generation' of virus particles.
Viruses cause disease. A virus may lie dormant for many years, yet, when it comes in contact with a suitable living cell, an amazing thing happens. The tiny virus particle 'hijacks' the host cell and 'forces' it to make hundreds of copies of itself. Eventually, the host cell dies and bursts open. The freshly made virus particles escape and go on to infect other cells. Among the many diseases caused by viruses are smallpox, polio, influenza, chickenpox, measles, German measles and colds.
Medically, virus infections are difficult to treat. Antibiotics are of no use against them. Immunization is often the only defense against serious viral diseases.
The steps involved with the virus 'hijacking' the host cell are:
1. The virus particle attaches itself to a suitable host cell. It injects its nucleic acid into the cell.
2. The nucleic acid takes control of the host cell. The cell stops its own activities and starts making only virus protein coats and nucleic acids.
3. The protein coats and nucleic acids join together to form hundreds of perfect replicas of the original virus inside the cell.
4. About thirty minutes after being infected the host cell may open and set free the new virus particles.
