Viruses are parasites which only live and reproduce inside cells of a living host.
1000 times smaller then bacteria and may be viewed only with an electron microscope, they enter the living plants or animal cells and reproduce within that cell, often destroying the host cell and must continually enter the new cells to survive, having no mechanism for transportation.
They depend on the air, water, insects, animals or humans to carry them from one host to another.
Some viruses may survive away from the host for hours or days when nutrients are high in the environment, such as blood, dead skin tissue and body wastes, causing diseases such as smallpox, influenza, rabies, measles, mumps, polio, hepatitis, H.I.V (aids), E-coli and the common cold.
Parvo, canine distemper, foot and mouth diseases are viral diseases that effect animals. Viruses cause plant diseases affecting such crops as tobacco, vegetable, fruits and sugarcanes.