In everyday life, and from a practical viewpoint, the only significant difference between a virus and a bacterium is its effect on the human body. Yet in both structure and function, these microscopic entities have little in common.
For example, the virus is not actually a form of life. It is a single-celled microbe, and its size varies from around 20 to 250 nanometers, a single nanometer being one billionth of a meter. It consists of a small collection of genetic material encased in a protective coat. The virus requires a host cell in order to replicate its genetic material and thus reproduce. When the virus is present outside a living cell it is dormant, but when inside, it takes over the resources of the host cell and begins to reproduce more virus particles. Thus it is actually a parasite, feeding off its host. It has no independent life functions. Antibiotics are of no use when it comes to viral infections; anti-viral medication is needed. Vaccines consist of weakened strains of viruses
A virus functions by triggering a cell to engulf it. The virus then connects to the cell and releases its DNA into the host cell, taking over its reproductive function. During this process the virus overrides the host cell’s normal function, shutting down its production of protein and directing its energy towards the production of new virus material. It is the host cell that actually does the work, while the virus directs the operation.
On the other hand, bacteria are single-celled living organisms that manufacture their own DNA and can be found virtually anywhere and everywhere: in the air, the soil, in water, and either within or upon plants and animals. All bacteria are surrounded by a cell wall and have the ability to reproduce independently.
Some bacteria are beneficial to humans and carry out functions such as the making of vitamins, the breaking down of garbage and the maintenance of the atmosphere at an optimum level for human life. The human mouth alone contains at least 500 species. Some bacteria present in the human body are able to prevent infections and produce substances such as Vitamin K. Good bacteria live symbiotically with humans, animals and plant life. Some produce energy by photosynthesis; others need to take in nutrients in order to survive and multiply. Bacteria are present in the stomachs of cows and sheep, enabling them to digest grass. Other bacteria are used in the production of yogurt and cheese. Some can be genetically altered in order to produce human proteins such as insulin for diabetics.
On the other hand, harmful bacteria cause infections including streptococcal throat, and infections of the urinary tract, ear and sinuses. Some bacteria can be parasitic and these invade cells rather than causing damage from the outside. A bacterial infection that is common in hospitals is caused by staphylococcus aureus (golden staph), a bacterium which is round in shape. Other bacteria come in different shapes, Escherichia coli being rod-shaped, Spirochetes spiral in form, and Clostridium tetani shaped like a drumstick.
So bacteria may be beneficial or harmful. However, viruses always cause harm. Diseases caused by viruses include AIDS, cancers, influenza and smallpox. The virus is sub-microscopic in size, being 10,000 times smaller than a bacterium. The greatest difference between the bacterium and the virus is that the former can usually multiply on non-living surfaces while the latter must have a living host in order to reproduce. Antibiotics can kill bacteria but have no effect on viruses. To complicate the picture, however, there are some infections (such as pneumonia, meningitis, diarrhea) that can be caused by either viruses or bacterium. A sample then needs to be taken for microbial culture in order to discover the source of the infection.