Bacteria are single-celled organisms called prokaryotes. Viruses are fragments of biomolecules that cannot reproduce independently. Both groups can be pathogenic or 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 useful species have played vital roles in the development of humanity as a species and of human social organization. Prokaryotes are divided into eubacteria and archaebacteria. Eubacteria are considered bacteria, and will henceforth be referred to as such. Archaea lived in more extreme environments, are older than eubacteria, and have enough chemical differences to be distinct from bacteria. Prokaryotes are single-celled organisms that reproduce by fission and conjugation, exhibit high 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 absence of membrane-bound organelles, such as lysosomes or endoplasmic reticulum. Bacteria have three types of shapes: shell, bacillus and spirilla (Starr, C., et al, 2004). The cocci are sphere-like and approximately 0.5 to 1 µm long, the bacilli are rod-like and 0.5 to 1.0 µm wide by 1.0 to 4.0 µm long, and the Spirilla are spiral-shaped and vary from 1 µm to over 100 µm in length. (Elert, G, 2006.) Designations of this type can be made more precise by adding morphemes at the beginning of the word, as in the case of diplococci, which refers to a pair of spherical bacteria. Gram-negative bacteria have an additional layer beyond the cell wall, called the 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). E. coli fact sheet. 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 pathogenic bacteria. Accessed January 14, 2010 at: http://www.mgc.ac.cn/VFs/main.htmStarr, C. and Taggart, R. (2004). Biology: The unity and diversity of life. United States: Wadsworth Group. University of Arizona. (2004). Infectious disease. Accessed January 14, 2010 at: http://microvet.arizona.edu/Courses/MIC419/Tutorials/infectiousdisease.htmlWorld Health Organization. (nd). Smallpox. Accessed January 14, 2010 at: http://www.who.int/mediacentre/factsheets/smallpox/en/