An infectious disease occurs when a pathogenic organism causes inflammation or organ dysfunction. This may be caused directly by the virus itself, as when the etiologic agent multiplies in the coordinator, or indirectly as a result from the host’s inflammatory response. Numerous infections are subclinical, not producing any obvious manifestations of illness.

To cause overt virus, all microorganisms should go via the following stages: The microorganism should (1) come across the host, (2) gain entry into the coordinator, (3) multiply and spread from the website of entry, and (4) trigger host tissue injury, either directly (eg, cytotoxins) or indirectly (host inflammatory response).

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