Bacteria are single-celled organisms called prokaryotes. Viruses are pieces of biomolecules that cannot reproduce independently. Both groups can be pathogenic or they can be beneficial: sometimes, the same species or variety can be both, depending on the circumstances (Starr, C. and Taggart, R., 2004). Throughout history, pathogens and more beneficial species have played vital roles in the development of humanity as a species and the human social order. Prokaryotes are divided into eubacteria and archaea. Eubacteria are considered bacteria and will be referred to as such from now on. Archaea lived in more extreme environments, are older than eubacteria, and have enough chemical differences to be distinguished from bacteria. Prokaryotes are single-celled organisms that reproduce through fission and conjugation, have great metabolic diversity, have a single chromosome arranged in a circle called a plasmid without a nuclear membrane, and tend to have cell walls. One of the most notable features is the lack of membrane-bound organelles, such as lysosomes or the endoplasmic reticulum. Bacteria have three types of forms: coccus, bacillus and spirillo (Starr, C., et al, 2004). The cocci are sphere-like and are 0.5 to 1 µm long, the bacilli are rod-shaped and measure 0.5-1.0 µm wide and 1.0-4.0 µm long, and the spirilli they are spiral-shaped and vary from 1 µm to over 100 µm in length. (Elert, G, 2006.) Designations of this kind can be made more precise by adding morphemes to the beginning of the word, as in the case of diplococcus, meaning a pair of spherical bacteria. Gram-negative bacteria have an additional covering beyond the cell wall, called a capsule. These capsules contain lipopolysaccharide, which is toxic and causes...... middle of paper ......ar Expressions. (2009). Structure of bacterial cells. Retrieved January 14, 2010, from: http://micro.magnet.fsu.edu/cells/bacteriacell.htmlThe Oregon Health Services. (2001). Fact sheet on Escherichia coli. Retrieved January 14, 2010, from: http://www.oregon.gov/DHS/ph/acd/diseases/ecoli/facts.shtml#commonState Key Laboratory for Molecular Virology and Genetic Engineering. (2009). Virulence factors of bacterial pathogens. Retrieved January 14, 2010, from: http://www.mgc.ac.cn/VFs/main.htmStarr, C. & Taggart, R. (2004). Biology: the unity and diversity of life. USA: Wadsworth Group. University of Arizona. (2004). Infectious disease. Retrieved January 14, 2010, from: http://microvet.arizona.edu/Courses/MIC419/Tutorials/infectiousdisease.htmlWorld Health Organization. (n.d.). Smallpox. Retrieved January 14, 2010, from: http://www.who.int/mediacentre/factsheets/smallpox/en/