These viruses may spread through the air when people cough or by direct contact. Risk factors include exposure to tobacco smoke, dust, and other air pollutants. A small number of cases are due to bacteria such as "Mycoplasma pneumoniae" or "Bordetella pertussis". Diagnosis is typically based on a person's signs and symptoms. The color of the sputum does not indicate if the infection is viral or bacterial. Determining the underlying organism is usually not required. Other causes of similar symptoms include asthma, pneumonia, bronchiolitis, bronchiectasis, and COPD. A chest X-ray may be useful to detect pneumonia. Another common sign of bronchitis is a cough which lasts ten days to three weeks. If the cough lasts for longer than a month, it may become chronic bronchitis. In addition, a fever may be present. Acute bronchitis is normally caused by a viral infection. Typically, these infections are rhinovirus, parainfluenza, or influenza. No specific testing is normally needed in order to diagnose acute bronchitis. One form of prevention is to avoid smoking and other lung irritants. Frequent hand washing may also be protective. Treatment for acute bronchitis usually involves rest, paracetamol (acetaminophen), and NSAIDs to help with the fever. Cough medicine has little support for its use, and is not recommended in children under the age of six. There is tentative evidence that salbutamol may be useful in treating wheezing; however, it may result in nervousness and tremors. Antibiotics should generally not be used. An exception is when acute bronchitis is due to pertussis. Tentative evidence supports honey and pelargonium to help with symptoms. Getting plenty of rest and drinking enough fluids are often recommended as well. Chinese medicinal herbs are of unclear effect. Acute bronchitis is one of the most common diseases. About 5% of adults are affected, and about 6% of children have at least one episode a year. It occurs more often in the winter. More than 10 million people in the US visit a doctor each year for this condition, with about 70% receiving antibiotics which are mostly not needed. There are efforts to decrease the use of antibiotics in acute bronchitis. Acute bronchitis is the most common type of bronchitis. Chronic bronchitis is defined as a productive cough that lasts for three months or more per year for at least two years.