Acute bronchitis is an inflammation of the large bronchi (medium-sized airways) in the lungs that is usually caused by viruses or bacteria and may last several days or weeks. Characteristic symptoms include cough, sputum (phlegm) production, and shortness of breath and wheezing related to the obstruction of the inflamed airways. Diagnosis is by clinical examination and sometimes microbiological examination of the phlegm. Treatment for acute bronchitis is typically symptomatic. As viruses cause most cases of acute bronchitis, antibiotics should not be used unless microscopic examination of Gram stained sputum reveals large numbers of bacteria.