A virus is smallest living organism. It is an organism that can CANNOT live independently (like a bacteria or fungus). A virus cannot survive for very long (sometimes only minutes) without being inside a living cell. The virus needs another cell to live within and to use its intercellular components to synthesize copies of itself (replicate its DNA or RNA) and do build its protective protein coat (for when it is outside the living cell). Antibiotics do not harm a virus because it is inside the cell and it is for this reason that treatment for the "cold" for example, is mainly to help ease the symptoms of the illness rather than to kill the organism which causes a "cold".
What Do Virus Do?
All viruses only exist to make more viruses.
What protects things against viral infection?
Humans are protected in a couple of ways.
First, if a particular virus infects one or more cells of a given tissue in our body, the infection leads to the synthesis and secretion of substances called interferons. These proteins interact with adjacent cells which help adjacent cells become more resistant to infection by the virus. Sometimes, this resistance isn't quite good enough to prevent the spread of the virus to more and more cells, and we begin to feel sick (we are now experiencing a disease caused by the viral infection). Now however, the body's immune system takes over and begins to fight the infection by killing the virus on the outside of the cells, and kills the infected cells, too. The killing of the infected cells prevents the spread of the virus, since as was stated above, a virus requires a living cell in order for the virus to be able to replicate. Eventually, the virus will be completely removed, and we'll get over the illness. HIV is an exception to this situation because HIV infects cells of the immune system which are necessary to kill the infected cells. So, although HIV does not itself directly cause the condition known as AIDS, the eventual death of immune cells due to infection with HIV allows other infections to harm a person.
Recently, there have been agents designed in the laboratory and isolated from natural sources which are being used to fight certain viral infections. These agents are not called antibiotics however, since they are effective only for viruses and have not been isolated from other organisms capable of killing a virus. So far, no agents have been identified which are secreted by a cell which actually kills a virus. You may be familiar with the drug called Acyclovir which is used to inhibit the replication of Herpesvirus; and, AZT and HIV protease inhibitors which are used to inhibit the replication of HIV.