Viruses

  ** A virus is a particle of nucleic acid, protein, and in some case lipids that can reproduce only by infecting living cells. A virus infects ANY type of cell to reproduce. The word "virus" comes from the latin word poison, first used in English in 1392. ** ** A typical virus is composed of a core of either DNA or RNA surrounded by a protein coat. A viruses outer protein coat is called its capsid. Most viruses are so small, they can only be seen with the aid of a powerful electron microscope. **
 * Viruses **
 * The process of how most viruses infect a cell are quite similar. First the capsid includes proteins that enable a virus to enter a host cell. The capsid protein of a typical virus bind to the surface of a cell and "trick" the cell into allowing it inside. Then, once inside, the viral genes take over. The cell then transcribes the viral genes, putting the genetic program of the virus into effect. SOMETIMES, that genetic program may simply cause the cell to make copies of the virus, but it often destroys the host cell. **
 * There are 2 types of viral infections: lysogenic and lytic infections. In a lytic infection, a virus enters a cell, makes copies of itself, and causes the cell to burst. **

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 ===== In lysogenic infection, a virus embeds its DNA into the DNA of the host cell and is replicated along with the host cells DNA.



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Viruses cause human diseases such as polio, measles, AIDS, mumps, influenza, yellow fever, rabies, and the common cold. In most viral infections, viruses attack and destroy certain cells in the body, causing the symptoms of the disease. The best way to protect against most viral diseases lies in prevention, often by the use of vaccines.

==**Their are several types of different viruses and variations of composition of viruses** ==

**-** ** Oncogenic: ** Viruses that cause cancer in animals.  
 * -Retroviruses: ** Viruses that contain RNA as their genetic information
 * -Prions : ** Short for “ Protein infectious particles” ; Viruses which contain no DNA or RNA, only protein.