Pathogenicity refers to the ability of an organism to cause disease (ie, harm the host). This ability represents a genetic component of the pathogen and the overt damage done to the host is a property of the host-pathogen interactions. Commensals and opportunistic pathogens lack this inherent ability to cause disease. However, disease is not an inevitable outcome of the host-pathogen interaction and, furthermore, pathogens can express a wide range of virulence. Virulence, a term often used interchangeably with pathogenicity, refers to the degree of pathology caused by the organism. The extent of the virulence is usually correlated with the ability of the pathogen to multiply within the host and may be affected by other factors. In summary, an organism is defined as being pathogenic (or not), and depending upon conditions, may exhibit different levels of virulence.

Pasteur and Koch clearly demonstrated the relationship between the microbial world and disease. Two of the best available textbooks of medical microbiology, for example a pathogen is a member of a microbial species and that virulence defines the specially harmful propensities of strains within such a pathogenic species. These definitions have directed the attention of microbiologists, physicians, veterinarians, and plant pathologists to a select-number of microorganisms and viruses obviously involved in causing harm to higher forms of life. Once recognized, ingenious methods were invented to curtail harmful microbial activities in a specific host and to prevent the spread-out such organisms from one individual to another. These activities resulted in the invention of chemotherapeutic and antimicrobial agents, the most recent success in our fight against microbial incursions. But the microbial world, renowned for its refusal to read the literature and its unwillingness to obey the dictates of the lords and ladies of creation, has demonstrated with increasing frequency that there are exceptions.

A growing number of carefully designed antimicrobial agents does not prevent hospital-acquired infections. These infections are not the result of established pathogens endowed with special virulence attributes. Instead, they are caused by microorganisms, widely distributed in nature and without any property or principle that would signify potential harm to patients. Many of these microorganisms resist antimicrobial agents and they complicate the recovery of patients whose immunity has been embarrassed by disease or therapy; their ubiquity in nature usually does not lead to disease in healthy residents in-the community.

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Emilie Rose
Managing Editor
Microbiology: Current Research
Email: aamcr@alliedacademies.org