Launching with a brief description of smallpox (variola) and monkeypox, the Chapter dwells on the currently important vaccine-preventable diseases like chickenpox, measles, German measles, and mumps, providing such details as etiopathogenesis, clinical features, diagnosis, treatment, prophylaxis and complications. Thereafter, it takes up the acute flaccid paralysis with special reference to poliomyelitis and Guillain-Barre syndrome, emphasizing on the 4-prolnged strategy to for total eradication of polio globally. Then follows a lucid description of Infectious mononucleosis and its role in causation of the so-called “chronic fatigue syndrome” usually affecting adolescents and adults. Dengue and chikungunya, both usually caused by the bite of the Aedes aegypti mosquito, are discussed in details, including etiopathogenesis, clinical features, diagnosis, differential diagnosis, complications, treatment, prevention and prognosis. The slow virus infections and hydrophobia (rabies, usually left out in most textbooks (perhaps, by default, receive a brief though clear-cut treatment. Viral hepatitis receives a passing reference since it has been extensively dealt with in Chapter 25 (Pediatric Hepatology). All aspects of pediatric acquired immune deficiency syndrome (AIDS), aptly described as the 20th century plague spilling into 21st century in a big way and the worst amongst the most devastating diseases of the day, are accorded fine coverage with stress on the ongoing advances in the field, including vaccine development. Finally are discussed the respiratory syncytial virus infections and bird (avian) flu. The threat of the H5N1 virus as the likely cause of a forthcoming large epidemic across the world (pandemic) in humans is appropriately driven home.