Bacteria are a domain of prokaryotes, the earliest type of life on earth. These microorganisms can be traced back billions of years, and there are an unknown, but massive, amount of them on the planet today. The unifying quality of all bacteria is that they are single-celled, and the vast majority are so small they can only be viewed under a microscope, although there are a rare few that can just barely be seen with the naked eye. Most bacteria are harmless, but harmful bacteria, also known as pathogenic bacteria, can cause incredible damage to a person's body, including death.
The majority of bacteria are known as nonpathogenic bacteria, and they comprise more than two-thirds of all those found on Earth. Nonpathogenic bacteria are found on and in everything, from skin, to water, to human stomachs. They actually are necessary to the healthy functioning of any life form, assisting with vital functions like digestion. These bacteria are constantly moving through the human body's systems, and cause no harm, except in very rare circumstances.
The term pathogenic means disease carrying, and pathogenic or harmful bacteria are often referred to simply as germs. They are generally classified based on their shape, with three main groupings: coccus, bacillus, and spirillium. The cocci can be further sub-divided into three groups: streptococci, diplococci, and staphylococci.
Each of these three main groupings of bacteria are associated with their own set of symptoms and ailments. The cocci, for example, produce pus and grow in larger groupings. They cause all sorts of pustules and boils in the body, and so are quite distinctive when they infect a wound or region of flesh. Bacilli are responsible for a number of much more serious diseases, most notably tuberculosis. Spirilla cause other serious diseases, including syphilis.
As single-celled organisms, harmful bacteria are able to reproduce quite rapidly. Through a process called mitosis, they split into two identical copies of themselves. These copies can then further split into two more copies, and so on, quickly enlarging a population to staggering numbers. An average bacterium can, within a 24 hour period, create many millions of copies of itself, overrunning an organism. Bacteria’s ability to reproduce on its own is one of the key differences between them and harmful viruses. Viruses are also pathogenic, but as they are much, much smaller than bacteria, they need some sort of a host to reproduce, making them slower and more awkward at reproduction.
There are many types of harmful bacteria, some of which are absolutely deadly, while others only cause minor illness. One of the most dangerous is Clostridium botulinum, which causes botulism; it can be found in food and can cause death even in incredibly small amounts. Another is Yersinia pestis, which caused the bubonic plague, and which still afflicts small portions of the world’s population. A less-dangerous, but still inconvenient, form is Campylobacter, which is responsible for most cases of food poisoning, causing severe intestinal discomfort and often vomiting. Other bacteria may be an inconvenience if treated, but deadly if left on their own, such as the different types of Streptococcus that are responsible for strep throat and pneumonia.