When physicians diagnose someone with walking pneumonia, they are usually referring to an infection with an organism called Mycoplasma pneumoniae. (Apart from being a pain, mycoplasmas are kind of cool. They are the smallest self-replicating biologic systems known.) Mycoplasma is a major cause of respiratory infections in school-aged children and young adults. It is most common between the ages of 5 and 15, accounting for 70% of pneumonias in children aged 9 to 15. Mycoplasma can be contagious with close contact. It usually requires prolonged contact to catch the disease. Epidemics have been reported in situations such as summer camps and boarding schools. The disease spreads through breathing air coughed by someone who is infected. Since the cough is often worse at night, people sleeping in the same room are most vulnerable.