This chapter focuses on community-acquired pneumonia (CAP) in adults and children, its causes in various age groups, indications for hospitalization and the challenges that clinicians face in making an accurate diagnosis despite the new and emerging diagnostic tests. Pertussis cases occur year round; neither infection nor immunization provides lifelong immunity. Lack of natural booster events and waning immunity since the most recent immunization, particularly when a cellular pertussis vaccine is used for the entire immunization series, are responsible for increased cases reported in school-age children, adolescents and adults. CAP is a syndrome in which acute chest infection develops in persons who have not been hospitalized recently and have not had regular exposure to the healthcare system. Apart from the clinical findings, a chest radiograph should be obtained in all patients. A repeat chest radiograph in 48 hours may show airspace diseases as well. On the basis of moderate evidence, fever and tachypnea are the most sensitive clinical signs of pneumonia, particularly after the first 3 days of illness. On the basis of moderate evidence, blood cultures may recover the causative organism in children hospitalized with severe pneumonia.