virus

A virus is a minute infectious agent (about 100 times smaller than bacteria) that cannot be seen under a light microscope.  They lack independent metabolism, are only able to replicate within the host cell, carry either DNA or RNA as their genetic material, and are protected by a protein coat.  The body's immune system is usually able to eliminate harmful viruses and vaccines provide immunity to a particular viral infection.  Antibiotics have no effect on viruses but antiviral medications suppress replication.

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