*3.6.4 Pulmonary drug delivery*

The treatment of lower respiratory infections is difficult due to the presence of microbes deep in the respiratory system. Nanoparticles as a targeted drug delivery system to the site of infection have been actively applied to enhance antimicrobial drug resistance, sustain drug release, and increase bioavailability. Importantly, nanoparticle systems that contain multiple antimicrobial substances can be used [51, 52].
