Walking pneumonia, (also referred to as atypical pneumonia) results from an infection in the lungs with signs and symptoms that often resemble other respiratory infections, quite similar to a cold. It is fairly common throughout the United States and is caused by a bacterium called Mycoplasma pneumonia. There are different kinds of pneumonia representing varying degrees of severity, but walking pneumonia falls into the least serious kind. Many people may think they just have a viral respiratory infection and don’t realize they have it.