A virus is a small amount of infectious genetic material that causes disease. There are many different types of viruses. They differ in size, shape, type of nucleic acid, type of host and disease they cause. Virus particles are too small to be seen without the aid of an electron microscope capable of magnifying objects more than 100,000 times actual size.
All types of organisms are subject to infection by viruses. Viruses enter and take control of the cells of their hosts and direct their own reproduction using mostly host enzymes, energy and materials such as amino acids and bases. A virus consists of a nucleic acid (RNA or DNA) surrounded by one or a few specific proteins. The protein component of the virus is often referred to as the ‘coat protein’ or ‘capsid protein’. Viral proteins including proteins key to viral reproduction, movement of virus within the host, spread to noninfected hosts and symptoms are coded for the virus nucleic acid. Viruses contain only enough nucleic acid to code for typically 4-10 proteins. They fullfill their needs for survival by genetically directing the host to use its resources for the reproduction of the virus. In just a few days of infection, millions of virus particles are produced per infected cell.
Viruses are obligate parasites in that they are completely dependent on hosts for their reproduction and have adapted efficient and specific ways to move within a plant and spread between plants by interacting with vectors, pollen, seed and tubers to insure their survival. They have the ability to adapt to new hosts and environments because they readily mutate during reproduction.