The pathogenesis of viral infections can be defined as how viruses cause disease in humans, animals, plants, or bacteria. Most knowledge of the pathogenesis of viral infections is based on experimental studies in laboratory animal models to understand natural infections in humans.

Viruses are one of the infectious agents with very small size (diameter from 20-300mm), simple structure but the ability to cause disease in humans is extremely fast. Viruses have the basic characteristics of an organism but they do not have a cellular structure, cannot reproduce themselves and do not have metabolism.