Most acute respiratory diseases are viral infections. They may be complicated by bacterial superinfections in which a bacterial infection develops after a viral infection. In such a situation, however, an illness would be of longer duration and not be considered acute. Acute respiratory infections (ARIs) are the most frequent illnesses experienced by most people globally. Young children have between five and seven of these illnesses per year, with a general decrease in frequency with increasing age. Adults will have about two such illnesses per year. It has been shown that among adults women have a higher frequency of respiratory disease than men do, and women who do not work outside the home have more frequent illnesses than those who do. This indicates that children are responsible for spreading illnesses in the family. Further evidence for this conclusion comes from the seasonal occurrence of the illnesses. While these acute infections occur year-round, in the temperate zones most begin in the colder season, and the first peak of illnesses follows the opening of schools by about two weeks.
Infectious Agents Causing Common Respiratory Infections
Acute Respiratory Disease, Pharyngoconjunctival fever, pneumonia, etc.
One reason why there are so many acute illnesses experienced throughout life is the large number of viruses that can cause the common cold and other similar syndromes (see Table 1). In addition, when infection with one of these agents occurs it does not result in lifetime immunity. Since the actual infection is on the surface of the respiratory tract rather than disseminated in the blood, it is much more affected by secretory IgA antibodies than circulatory IgG antibodies. As a result, immunity produced by past experience with the same virus is relative rather than absolute. This means there may be protection from reinfection over a period of months rather than years, and if reinfection occurs, it may produce an inapparent infection or milder illness than would be experienced without prior infection. This situation is most dramatic with agents such as respiratory syncytial virus (RSV) and parainfluenza viruses. The initial infection can produce potentially life-threatening illnesses in young children, such as bronchitis or croup, but later in life reinfection will produce only a common cold-like syndrome.
ARNOLD MONTO, The Gale Group Inc., Macmillan Reference USA, New York,