A minute infectious agent to small to be seen under the ordinary laboratory light microscope but visible under the magnification of the electron microscope. This is surrounded by a protein coat which may be multilayered. Some viruses, such as herpes, have a third coating which is membranous in nature and may be taken from the animal cell just infected. Viruses may infect animals, plants, or bacteria. They are not susceptible to the usual antibiotics, such as penicillin, but may be inhibited by special antimetabolites or other antiviral agents which have little to no effect on bacteria.
Viruses are incapable of growing anywhere but within living cells.
Those blood corpuscles responsible for maintaining the body's immune surveillance system against invasion by foreign substances such as viruses or bacteria. White cells become specifically programmed against foreign invaders and work to inactivate and rid the body of a foreign substance.