The most common form of pneumonia in infancy is bronchopneumonia, also known as lobar pneumonia, which is the most common form of pneumonia in the pediatric population. It is commonly caused by group B streptococci and is part of early onset sepsis, which is usually severe, with lesions involving both lungs and showing diffuse foci of infection. Gram-negative enterobacteria usually cause nosocomial infections, so they are usually detected in the first week of life. Cytomegalovirus-induced pneumonia is often part of a systemic cytomegalovirus infection, and other signs of congenital infection are usually present. Pneumonia caused by Chlamydia trachomatis is caused by an infection of the mother’s reproductive tract, with no fever or a low-grade fever and a violent cough that resembles a whooping cough. Pneumonia caused by respiratory syncytial virus has a peak age of onset between 2 and 7 months after birth and is characterized by wheezing and profuse runny nose, with onset in winter or early spring. Pneumonia caused by streptococci is probably the most common cause of bacterial pneumonia. Pneumonia caused by pertussis usually causes pneumonia in severe cases and bronchiolitis in mild cases. Severe pneumonia caused by Staphylococcus aureus is characterized by the simultaneous presence of infiltrative foci, pulmonary pustules, pulmonary blisters, abscesses, and pneumothorax.