Epidemiology of respiratory infectious diseases.

Respiratory infections are a frequent burden to health despite the fact that cost-effective methods for their prevention and cure are available. Acute respiratory infections in children under 5 years of age are the most frequent cause of death from lung disease globally, causing more than 4 million deaths annually. Tuberculosis is the most frequent cause of death from a single pathogen in persons aged 15 to 49 years (a total of 2 million to 3 million deaths annually). Respiratory infections are the most frequent complications of immune deficiency (whether due to HIV infection or induced by chemotherapy). Where a "carrier state" occurs (as with many bacterial pathogens), the level of immune function is the key determinant in appearance of disease. Where there is no carrier state (as with many viruses), exposure is the key determinant. Characteristics of the pathogen, including virulence and bacterial load where there is a carrier state, also determine the probability of respiratory infections. Modifiers of these determinants include allergy and toxic exposures including tobacco smoke and ambient pollution.