Antibiotic therapy is started with a broad-spectrum antibiotic because broad-spectrum antibiotics are effective against many gram-positive and many gram-negative bacteria.
Azithromycin is a semisynthetic broad-spectrum antibiotic that can be used as an alternative to penicillin, is broader in range than erythromycin, and has better tissue penetration. Dr. Bell most likely assumes that the bacterium causing Caleb's infection will be susceptible to azithromycin and that this drug should clear the infection.
