The diseases and disorders of the respiratory system can affect any part of the respiratory tract and range from trivial to life-threatening. The nasal passages and pharynx, for example, are targets for the viruses that cause colds. These viruses infiltrate and destroy the cells of the nasal passage membranes. The immune system fights back by increasing blood flow to the area, bringing numerous virus-attacking white blood cells to the scene; this causes the membranes to swell, resulting in the stuffy nose associated with colds. Mucous secretions increase in response to the viral attack, creating the runny nose typical of colds. The infection can spread to the sinuses, the membrane-lined cavities in the head, as well as the lower respiratory tract and the middle ear.
Acute respiratory distress syndrome is one of two human diseases caused by hantavirus. Dust containing virus-infected rodent feces becomes airborne and is inhaled. The virus embeds in the lungs where the infection begins. Flulike symptoms appear in about a week, followed by the collection of fluid and white blood cells in the lungs, causing respiratory failure, then death.