Microbes are tiny organisms that are too small to be seen without a microscope. They fall into four major categories: bacteria, fungi, protozoa, and viruses
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. These microbes often get a bad reputation when they are referred to as "germs" or "bugs," but while some microbes cause infectious diseases in humans, others are necessary for a healthy life
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. Scientists have uncovered evidence that microbes have been infecting plants and animals with infectious diseases since ancient times by observing prehistoric material such as human skeletons and mummified remains
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Bacteria
The group of microbes known as bacteria is single-celled organisms that fall into the category of life known as the prokaryotes. Prokaryotic cells do not contain a nucleus, but bacterial cells do contain DNA as their genetic material. Like other prokaryotic cells, bacteria often have a rough cell wall, plasma membrane, and ribosomes suspended in their cytoplasm. Many bacteria require oxygen and need food for energy
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. Bacteria are among the oldest living things on Earth, with scientists having found fossilized remains that date back more than 3.5 billion years
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. Bacteria can also survive in the most extreme conditions, from extremely cold to extremely hot areas. There are thousands of species of bacteria but a majority of bacteria fall into three basic shapes: rod-shaped bacilli, ball-shaped cocci, and spiral-shaped
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. Of the thousands of species of bacteria, less than one percent causes diseases in humans
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Viruses
It is often debated whether or not a virus is considered a living thing. Most notably, these functions include metabolism and reproduction. While viruses do contain their own genetic material (DNA or RNA), a virus is not made up of cells and cannot perform the basic functions of living things without hijacking a cell and taking control of its metabolism to reproduce
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. So is it alive? Or is a virus this strange particle that straddles a thin line between a living and nonliving thing? The question continues to be asked by scientists today. Viruses are basically small bundles of genetic material, either DNA or RNA, covered in a protein coat called a capsid
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. They can be rod-shaped, sphere-shaped, or icosahedral
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. Like other microbes, viruses can be found almost everywhere on Earth. They are the smallest microbe, and are basically found anywhere that there is a cell to infect; from bacterial cells to human cells. There are different viruses, each one behaving differently and very particular about what type of cell it hijacks and infects
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. When they attack, viruses attach to the outside of a cell, enter the cell or inject their genetic material, and then take over the metabolism of the cell and direct it to make more copies of the viruses that are released to infect other cells
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