What is the defenition of systemic viral infection?

Many viruses cause systemic infections. A systemic infection is one which may involves multiple systems (organs) in the body. For example the virus may enter the body through the respiratory tract and initially cause some minor upper respiratory symptoms but then it may enter the bloodstream and travel to other parts of the body such as the parotid glands or the liver or spleen and may present as a rash or cause liver or spleen enlargement or indeed it may enter into the central nervous system and give rise to meningitis or encephalitis. So this is what is meant by a systemic viral infection. Examples of some viruses that cause major systemic viral infections are Mumps, Measles, Rubella and Varicella Zoster (chickenpox/shingles).