Acute bronchitis usually has a cough that lasts around three weeks. In more than 90% of the cases the cause is a viral infection. These viruses may be spread though the air when people cough or by direct contac. Risk factors include exposure to tabacco smoke, dust, and other air pollution. Treatmen of acute bronchitis typically involves rest, paracetamol, and NSAIDs to help with fever. Chronic bronchitis is defined as a productive cough that lasts for three months or more per year for at least two years.