Zithromax is a label name for azithromycin. It is more active than other antibiotics against pneumococcal bacteria and bacterial flu strains, in particular Haemophilus influenzae, H. parainfluenzae. Viral infections like CV and other respiratory virus weaken the system and open the door for bacterial infections. Antibiotics fall in to specific categories of antibacterial effectiveness and into wide spectrum effectiveness. Some are better than others against specific bacterial strains.
Azithromycin also helps reduce inflammation in the air ways by reducing a particular immune response which leads to cytokine storm. (Look up Az impact on eosinaphuls)