Causes, diagnosis, and treatment of pharyngitis. Pharyngitis is a common disease of the respiratory tract that can be caused by several different viruses and bacterial organisms. Clinically speaking, the most important causative agent is group A streptococcus (Streptococcus pyogenes). Although rare, postpharyngitis complications arise as a result of disease caused almost exclusively by group A streptococcus. Because group A streptococcal pharyngitis usually responds well to antimicrobial treatment, it is important to diagnose it. Penicillin, erythromycin, and peroral first-generation cephalosporins have been documented to be effective. In addition to group A streptococcus, C. pneumoniae and M. pneumoniae have also been detected in patients with pharyngitis. The possibility of diagnosing these organisms is limited at the present. Clinical surveys are still needed, moreover, to evaluate the effect of antimicrobial treatment on the disease caused by these organisms. Although routine viral diagnostic methods do not help primary care physicians in treating patients with pharyngitis, information on bacteria and viruses in the immediate environment could prove to be of great help in daily clinical work. 