There are a number of differences between a virus and bacteria, or more appropriately, viruses and bacteria, including their relative sizes, reproduction methods, and resistance to medical intervention. Bacterium, the singular form of the word bacteria, is a one-celled living organism, with complete sets of both ribonucleic acid (RNA) and deoxyribonucleic acid (DNA) genetic codes. A virus is little more than a section of RNA or DNA covered by a protein shell. A bacterium, therefore, is at least a hundred times larger than a typical virus.
One major difference between viruses and bacteria is the method of reproduction. A bacterium is a completely self-contained and self-reproducing unit. When the time is right, a bacterium will split its DNA and RNA genetic material in two. Separate cell walls will build up around these two new bacteria, and this process will continue until thousands or millions of bacteria have formed. This is how strains of bacteria survive in almost every environment on Earth, including non-living surfaces like rocks or plastic.
A virus, on the other hand, cannot reproduce without a living host. A virus may lie dormant for thousands of years before finally coming into contact with a suitable host. Once it enters the body of a host, a virus uses leg-like appendages to clamp onto a cell and a spike or chemical coating to penetrate the cell wall.
Once inside a living cell, a virus replaces the cell's original DNA or RNA commands with its own genetic instructions. Those instructions are usually to make as many copies of the virus as possible. Once the individual cell has outlived its usefulness, it explodes and sends out thousands of copies of the original virus to other unsuspecting cells.
Ninety-nine percent of all known bacteria are considered beneficial to humans, or at least harmless. They spend their days breaking down organic matter and destroying harmful parasites. The small percentage of bacteria considered harmful to the human body, such as streptococcus and E. coli, are still performing the same functions as their less-toxic bacteria brethren.
Bacteria feed on the tissues of the human body and excrete toxins and acids. It is these toxins and irritating acids that cause many of the problems associated with bacterial infection. If the bacteria can be killed with antibiotics, the infections left behind should quickly clear up.
Most viruses, on the other hand, serve no beneficial purpose. Their sole mission in life is to create more viruses in order to assure survival of the strain. The deadly effect a virus has on its host is merely incidental.
When a virus enters the human body, it seeks out an acceptable host cell and seizes it without warning. Once the cell bursts, thousands of new viruses repeat the process on other healthy cells. By the time the body's natural defenses become aware of the invasion, the virus may have significant control over vital organs and tissues. Human Immunodeficiency Virus (HIV) and the Ebola virus are textbook examples of what dangerous viruses can do to a human body before they run their course.
Medical intervention and treatment is another major difference between viruses and bacteria. Bacteria are alive, which means they can be killed by some form of chemical agent. Antibiotics are chemical compounds that kill off bacteria by destroying their cell walls or neutralizing their ability to reproduce. The reason doctors prescribe lengthy rounds of antibiotics to patients is to create an environment in which bacteria cannot live. Although bacteria often eventually develop a tolerance for certain antibiotics, the effect is similar to using insecticide on insects.
Viruses, by comparison, are not considered living creatures. They cannot be "killed" in the same sense as antibiotics kill bacteria or insecticide kills insects. In fact, treatment of viral infections is often no treatment at all. The disease must run its course until the body can mount a successful defense on its own. Anti-viral treatments, when they exist, work on the principle of blocking the virus' own destructive methods. Either the RNA or DNA strand of the virus must be rendered harmless genetically, or the methods of breaking through a cell wall must be destroyed.
Anti-viral medications are not effective against all viruses, which is why certain diseases such as AIDS, HIV, and Ebola are still affecting millions of people world-wide. Scientists are still trying to understand the basic structure and genetic programming of viruses. Only by understanding how a virus works can a successful vaccine eventually be developed. Treating most bacteria-based diseases, on the other hand, can be a matter of finding the most effective antibiotic or using a broad spectrum approach.