The clinical outcome is improved if patients with chronic lung diseases such as cystic fibrosis are treated long-term with the antibiotic azithromycin. However, azithromycin treatment in patients with cystic fibrosis as recently associated with increased infection with nontuberculous mycobacteria. Now, researchers have confirmed that long-term use of azithromycin by adults with cystic fibrosis is associated with infection with nontuberculous mycobacteria and identified an underlying mechanism.