The summary in Table 1 could be used as a mental checklist for the pediatrician who examines a child with fever. Whether the pediatrician opts to "keep the rules" or appropriately decides to "break the rules," knowledge of the guidelines will help him to focus his approach and to adopt attitudes of caution in certain circumstances. The body of knowledge of infectious agents chemotherapeutic agents has burgeoned over the past 40 years; the rules have changed very little. Thus, the rules might also serve as standards against which "new discoveries" that dictate departure from an established mode of clinical practice would have to be weighed. The adage, "Name the bug before you choose a drug," is especially germaine to pediatrics. Potential pathogens or "bugs" continually change as the patient's age, exposure, and immunity change. The serious diseases they cause mandate that initial treatment be given with the best "drugs." The age-related causes of bacterial meningitis presented in Table 2 could serve as a primer for age-related causes of other invasive disease as well. For bone, joint, and soft tissue infection as well as for septicemia without a focus the age line for group B Streptococcus and H influenzae would be extended upward and S aureus would be added for all ages. Although the relative importance of each pathogen for each clinical entity might vary, therapeutic considerations would be appropriately served by a schema such as this. Unfortunately, the susceptibility of pathogens to antimicrobial agents will continue to change. Fortunately, new and potentially better therapeutic agents will continue to be discovered or invented. When new problems of antibiotic resistance emerge or when superior therapeutic modalities are proved, the pediatrician must be knowledgeable of such events and be prepared for change.