The ability of a disease agent to cause impairment or dysfunction in an animal is called virulence or pathogenicity . These terms are synonymous, and can be used to describe the effects of any type of disease, but generally are used in the context of infectious diseases. Virulence and pathogenicity can be measured in terms of host mortality, reproduction, and altered life history. High virulence can occur as a coincidental by - product of infection, or it can be adaptive (beneficial) to the infectious agent.
Coincidental virulence offers no adaptive advantage to the causative agent; thus, virulence may be viewed as simply an accident. Coincidental virulence is commonly seen in new or novel associations. Infectious diseases that have coevolved a relatively benign relationship with wildlife, for example, can be highly virulent to domestic animals or newly introduced wildlife because they have not had opportunity to evolve a relationship that benefi ts both the host and parasite. One - sided relationships favoring either the host or disease agent are not advantageous to the disease agent. In cases where the host is favored, the disease agent may be fully suppressed and not able to reproduce and spread to new hosts. On the other hand, relationships favoring the disease may result in premature death of the host, also resulting in failure of the disease to spread to new hosts. An example of coincidental virulence occurs with elaeophorosis, a nematode disease of wildlife. The nematodes are transmitted to wildlife by the bite of horsefl ies (Diptera: Tabanidae) but usually cause no harm to their normal host in western North America, mule deer ( Odocoileus hemionus ). The same nematode, when transmitted to abnormal hosts such as elk, Cervus elaphus ; moose, Alces alces ; white - tailed deer, Odocoileus virginianus ; or bighorn sheep, Ovis canadensis ; can cause disease. High virulence of infectious diseases originating with wildlife but adversely affecting humans and livestock is most often coincidental.
