Most infections and illnesses are caused by the pertinaciousness of bacteria in alocation and their interaction with humans. Bacterial infections are facilitated by a variety of virulence factors that fa­cilitate various aspects of their pathophysiology that are critical for disease in the host (Falkow, 1991). Membrane proteins and adhesins, for example, facilitate bacterial colonization, attachment, and inva­sion of host cells. Furthermore, microbial toxins cause tissue damage in the host, and bacterial cell wall machineriessuch as capsular polysaccharides impart resistance to the host immune system (Taylor & Roberts, 2005). Spore formation and biofilm developing ability are two further virulence features that aid pathogen persistence in severe environmental circumstances.

Replication of certain RNA viruses are different in that their genetic material could be used as mes­senger RNA to synthesized the virus proteins, with a particular viral RNA polymerase copying the RNA template to produce the genome for the newly synthesized viral particles. RNA viruses known as retroviruses transcribe the RNA genome into DNA using a special enzyme called reverse transcriptase. The DNA subsequently integrates into the genome of the host cell. Long dormant periods are common in these viruses, during which their genetic materials are reliably reproduced and spread to daughter cells every time the cell divides. A well-known illustration of a retrovirus is the human immunodeficiency virus (HIV), which causes AIDS.Viruses, like other infectious agents, create illness by interfering with normal cell activity. They accomplish this in a number of ways. Some viruses produce repressor proteins that prevent the creation of proteins, RNA, and DNA in the host cell. Cell membranes and lysosomal membranes may be weakened by viral activity, resulting in cell autolysis. Some viral proteins are harmful to cells, and virus-infected cells may be killed by the body’s immune system. Viruses are categorized with reference to their mechanism of replication, genome structure, chemical makeup, form and size. Many pleomorphic and filamentous viruses have helical morphology in their nucleocapsids. A helical array of capsid proteins (protomers) wraps around a helical filament of nucleic acid to form helical nucleocapsids. The nucleocapsids of many “spherical” viruses have an icosahedral shape. Recognition, classification and documentation are enhanced by the arrangement and number of capsomeres. An outer envelope is found on many viruses.