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Pathogenicity
This term refers to the ability of a parasite to gain entry into hosts tissues and bring about a change (anatomical or physiological) resulting in a change of health and thus disease. The word is derived from pathos (Greek) meaning suffering. The term pathogen has same root and refers to an organism able to cause disease i.e. possessing pathogenicity.

The symbiotic relationship between host and parasite is called parasitism. Parasites vary in their pathogenicity. Even in the same taxonomic species there may be non pathogenic, less pathogenic or much pathogenic strains. Some as parasites of cholera, typhoid fever, plague are known to cause serious human diseases, whereas others like common cold viruses are less pathogenic.

Several factors may alter the balance of microbes. Destruction of normal flora by indiscriminate use of antibiotics may lead to diseases.  A new concept has thus emerged in recent years. Traditionally microbiologists believed that organisms were either pathogenic or nonpathogenic.

This distinction has been blurred by the fact that normally benign organisms may be pathogenic when body defenses weaken or fail. The AIDS illustrates this concept very well.

It is now recognised that pathogenicity is a function of the aggressive nature of the parasite as well as the level of resistance in the host. Whereas pathogenicity is used in a qualitative sense, virulence used in quantitative. sense giving a measure of the extent of pathogenicity of a microorganism.