ABSTRACT
Inuenza or u is caused by three species of single-stranded RNA viruses: inuenza virus C, inuenza virus B, and inuenza virus A, which will be referred to in this book as types C, B, and A viruses (Figures 24.1 and 24.2). Humans are reservoir hosts for type C and type B viruses, and these are referred to as human inuenza viruses. Types C and B viruses rarely infect other species except that type B viruses are also found in marine mammals. Type C viruses are common in humans and usually cause no symptoms or only a mild illness. Type B viruses cause sporadic outbreaks of respiratory disease in humans, especially in children. Type A viruses infect birds, especially waterbirds, but some strains have developed the ability to infect humans and other mammals including horses, hogs, and marine mammals. Type A viruses cause u epidemics every winter that sicken and kill people. In some years, type A viruses mutate into a more lethal virus that can easily spread from one person to another. When this happens, a worldwide pandemic (i.e., a worldwide epidemic) may occur, resulting in great loss of life.
