Pathogenesis: Infection and the Spread of Viruses in the Body

At the molecular and cellular levels, viruses behave quite differently from bacteria and protozoa but to some extent, this distinction disappears when viruses are considered at the levels of the whole animal and the population. Viruses, like other infectious agents, enter the animal body through one of its surfaces. They then spread either locally on one of the body surfaces or through lymphatic and blood vessels to produce systemic infection. To infect its host, a virus first attaches to and infects cells of one of the body surfaces: the skin, the mucosa of the digestive, respiratory, or urogenital tract, or the conjunctiva. Parenteral injection, either by needle or by the bite of an arthropod or vertebrate, bypasses the body surfaces. A lesion may be produced at the site of entry, although invasion may occur without the development of any local evidence of infection. The infection may remain localized in the skin, respiratory tract, or digestive tract or it may spread through the animal via the lymphatics, blood vessels, or nerves.