Viruses are submicroscopic and can only be examined in detail by using an electron microscope. Viruses do not grow, eat, or have measurable respiration; therefore, they do not require nutrients. Viruses have a method of replication that is different from that of all other organisms. They are capable of directing a host cell to make virus components. These components, or parts, are put together as on an assembly line in a factory.
Viruses contain either RNA or DNA, but not both (bacteria have both). The nucleic acids (RNA and DNA) are enclosed in a protein coat called a capsid. Viruses are divided into two groups for classification: those with RNA and those with DNA.
Viruses do not duplicate themselves outside of a cell. They are able to replicate only within cells. The DNA or RNA is able to direct the cells to make more viruses. Viruses cannot be cultured on artificial laboratory media of any kind. Some viruses can be cultured on a living chick embryo. Still other viruses can be cultured on liver cells and other types of cells in tissue culture.
Viral diseases are among the worst diseases that affect humans. Epidemic influenza (flu), colds, German measles, mumps, polio, rabies, chicken pox, and many other diseases are caused by viruses.
Viruses vs. Bacteria
Viruses are much smaller than bacteria. They are the simplest and smallest life form known. Unlike bacteria, viruses are not true living organisms because they are not made of cells. Viruses are bundles of genetic material or information enclosed in a capsid. They are unable to reproduce themselves and must rely on a host cell for replication. Viruses are like cell hijackers. They inject their genes (either RNA or DNA) into the nucleus of a cell and make it create new viruses. Eventually the build-up of viruses causes the host cell to burst, releasing the new viruses.
Like bacteria, viruses are found just about everywhere on earth. They are able to infect every form of life on the planet, from blades of grass to humans. Although viruses cannot survive very long without a host, they can remain inactive on nonliving surfaces such as countertops and doorknobs. Once picked up by a living host, the virus becomes active. Viruses can be contracted through air, water, bodily fluids, and direct contact. Unlike bacterial infections, many viral infections cause no symptoms. Often people do not even know they are infected. Viral infections cannot be treated with antibiotics like most bacterial infections can be. In fact, most viral infections have no treatment at all. Instead, there are preventative methods for blocking a virus.