Final answer:
Azithromycin is the antibiotic commonly used for both treatment and postexposure prophylaxis of pertussis, and it is effective when administered during the early stages of infection.
Step-by-step explanation:
The antibiotic commonly used for the treatment and postexposure prophylaxis of pertussis, a highly contagious respiratory disease, is azithromycin. This antibiotic is effective because it targets bacterial ribosomes, which are critical for protein synthesis in bacteria. When administered during the early stage of infection, azithromycin can reduce the duration of symptoms and prevent transmission to others. Pertussis is generally a self-limiting disease, but antibiotic therapy with erythromycin or tetracycline is most effective at the earliest stages. Vaccination with DTaP or Tdap remains the best approach for control and prevention of pertussis.