Concept preview

What you will be able to do

1. 01Describe the basic structure of a virus, including the capsid and genome.
2. 02Compare a bacteriophage with an enveloped virus.
3. 03Explain how small viruses are relative to cells and why host specificity exists.
4. 04Evaluate the arguments for and against classifying viruses as living.

A virus is the strangest thing biology studies: a scrap of genetic material in a protein shell, far too small to see with a light microscope, that can do absolutely nothing on its own. It cannot grow, cannot make energy, cannot even reproduce by itself. Yet inject it into the right living cell and it hijacks that cell's machinery to make thousands of copies of itself, often destroying the cell in the process. Viruses are obligate intracellular parasites — 'obligate' because they have no choice, and 'intracellular' because they only work inside a host cell. They sit right on the border between the living and the non-living.