The normal lower respiratory tract is free of viable bacteria, whereas the throat and nasopharynx become colonized by several common species of bacteria soon after birth. Moreover, in patients with physical and roentgenographic signs of lower respiratory tract pathology, there often are great discrepancies between the organisms seen in smears or grown in cultures of exudates obtained from the affected sites in the lungs and in those of sputum or secretions from the throat or nasopharynx when similarly examined. In infants and children, and even in many adults, particularly women, sputum representative of exudate from the lower respiratory tract often is not available or, when raised, is mixed with secretions from the upper respiratory tract.

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