The main systemic signs and symptoms of severe bacterial infection are fever and weakness. The most characteristic laboratory finding is leukocytosis, with an increase in number and immaturity of the neutrophils. However, in proven infection sometimes leukocytosis may be minimal or even absent, and occasionally fever may be minimal or may not be present (page 59). This is not frequent but happens more often in infants and the elderly. It also happens more frequently in debilitated persons, especially those with other severe diseases that may impair the ability of the body to respond normally to infection. Overwhelming infection (e.g., massive pneumonia or septicemia) may have normal WBC count or even leukopenia.