Bacteria (plural of bacterium) are single-celled (one-celled) microorganisms. They are invisible to the naked eye (an average 1,000 nanometers long) and can only be seen with the aid of a light microscope. Structurally, their shape is determined by the shape of the rigid cell wall that covers the entire structure of the bacteria. Due to the fact that bacteria has a cell wall, they may appear in one of three shapes : bacillary (rod-shaped), coccal (spherical or ovoid) or spirochetal (spiral/helical - corkscrew). They usually have tiny flagella (a tail like appendage that they use to swim around in their environment). Bacteria do not contain a nucleus or any other membrane-bound organelles and are therefore classed as prokaryotes. They can reproduce independently (self reproduce) by a process called binary fission where the parent cell (bacterium) divides (splits) into two new cells (bacteria) called daughter cells. This process is repeated continually where thousands or millions of bacteria are formed.

Most bacteria are harmless and some are even considered beneficial to humans for example, gut flora that is found in the digestive tract helps with the digestive process of food. However, there are others that live in and reproduce in warm, moist environments in the body where they grow quickly and feed on the tissues of the organism body, causing an infection by their excretion of toxins and acids that they produce (e.g. streptococcus and E. coli). Diseases caused by bacteria usually cause inflammation, swelling and pain from nerve irritation and fever - caused by increased body temperature by the body fighting the disease. There are different species of bacteria that are responsible for many plant and animal diseases, as well as, playing a very important role in the environment where they are also responsible for decay, fermentation and nitrogen fixation. Some examples of bacterial disease are: Cholera spreads by the ingestion of contaminated water and food containing the vibrio cholera agent by excrement of an individual with the disease; Syphilis spreads by sexual intercourse of an infected person to the sexual partner or from an infected mother to her fetus containing the disease agent Treponema pallidium. Other bacterial diseases include; Dysentery, Leprosy, Plague and Scarlet Fever just to name a few. Since bacteria can be killed with antibiotics, this means that bacterial infections can be treated with antibiotics.

Bacteria can be found inhabiting almost every environmental condition on earth. They are found in plants and animals, soil, water, air, rocks, permanently frozen ice of Antarctica and volcanic cracks miles under the surface of the ocean.

Bacteria Classification
Bacteria are generally classified into two main groups based on their color examined under a microscope when stained with a special (dye) stain in a laboratory. These two groups are "Gram-positive" (blue) and "Gram-negative"(pink). This laboratory procedure is called Gram Stain and based on differences in cell wall thickness with varying glycosaminopeptide and lipoprotein compositions. Gram-positive bacteria are a rare group of bacteria in comparison to the Gram-negative bacteria. They are easily treatable with the antibotic penicillin. They cause infection through the damaging of the host tissue by penetrating deep into tissue or by releasing toxic (poisonous) substance in the surrounding tissue cells. An example of this bacterial infection is anthrax. Gram-negative Bacteria E.g. Spirochetes, spiral and curved, aerobic and facultatively aerobic rods, obligate anaerobes, aerobic and anaerobic cocci, sulfate and sulfur-reducing, rickettsias, clamydias, mycoplasmas. Gram-positive Bacteria E.g. Cocci, endospore-forming and nonsporing rods, mycobacteria, nonfilamentous actinomycetes

Viruses
Viruses are ultramicroscopic (20 to 300 nm in diameter) organisms which are smaller than bacteria and can only be seen using an electron microscope. They are acellular (not cellular) and are structurally very simple. Structurally, they are composed of genetic material containing one type of nuclei acid molecule (either DNA or RNA) surrounded by a protective protein coat. They can be rod-shaped, sphere-shaped, or multisided in appearance. They cannot reproduce independently (they lack the means for self-reproduction) outside a host cell and depend on their host cellular system to reproduce. This makes them parasitic. Virus outside of a living cell is dormant (inert) and are thus, considered living only when they invade and multiply within a host cells that they infect. This means that they cannot reproduce without being in a living cell. This they achieve however by using their host chemical machinery to keep itself alive and to replicate itself. The virus DNA or RNA enters cells and use its DNA or RNA (genetic code) to make a copy of itself (produce more viruses), similar to what the ordinary cell would do by using the host cell's metabolism. This means that the host body cell is tricked (now controlled by the virus) into making many copies of the virus inside the cell host cell thus, killing the cell where the viruses then enter (infecting) other cells to repeat the process. Virus can also reproduce and mutate into new strains (new type virus) in their host, making it much difficult to treat the infected person (the host) from one person to the next. Inside the body viruses produce toxins (poisons) that can cause rashes, aches and fevers.

Virus is very difficult to kill and cannot be killed with antibiotics like bacteria. Some virus can be prevented through vaccination where a harmless variant of the virus is made and use to inoculate a person or animal to stimulate an immune defense against the "real" virus. e.g. smallpox. Some examples of viral diseases are: Aids - transmitted through bodily fluid from an infected human to another through semen, vaginal secretion, blood and the sharing of needles by the disease agent HIV 1 and HIV 11 and Smallpox - through human to human contact through sneezing and coughing by the disease agent variola. Others include Influenza, Yellow Fever and Lassa Fever just to name a few.

Virus Classification
Viruses are usually classified (grouped) according to its phenotypic characteristics such as; its size (morphology), the type of nucleic acid they contain (RNA or DNA), presence or absence of an envelope, the structure of the capsid (symmetry of the capsid), the number of protein subunits it contains, host species (organisms they infect), mode of replication and immunological characteristics (the type of disease they cause). One common method currently used for the classification of virus is the Baltimore classification system. This method groups viruses based on their mode of replication and genome structure (genome type). Viruses are classified in one of the seven groups where each, grouped (1-7), are designated by Roman numerals.

DNA viruses

• Group I: double-stranded DNA viruses. E.g. herpes viruses, chickenpox and smallpox.

• Group II: single-stranded DNA viruses. E.g. bacteriophage M13.

RNA viruses

• Group III: double-stranded RNA viruses

• Group IV: positive-sense single-stranded RNA viruses. E.g. hepatitis C virus, rubella virus and SARS virus and yellow fever virus,

• Group V: negative-sense single-stranded RNA viruses. E.g. Ebola, Marburg viruses, measles, mumps and rabies.

Reverse transcribing viruses

• Group VI: reverse transcribing RNA viruses. E.g. HIV

• Group VII: reverse transcribing DNA viruses. E.g. hepatitis B

Additional information and method guidelines in naming and classifying viruses is set out by the International Committee on Taxonomy of Viruses.

There is also the classification of Subviral agents. These are viral protein or other substance smaller than a virus and having some of the properties of a virus. Subviral Agents: Satellites, Viroids, and Agents of Spongiform Encephalopathies.