You first have to know that pneumonia is the condition wherein the lungs become inflamed and swollen. There are different reasons for this. The lungs may become infected because of germs, or they may get infected because of infection.
When you say atypical pneumonia, this is a type of pneumonia that is caused by bacteria, but for a strange reason, they are caused by bacteria that are not too common. If you would check out more differences between the two, atypical pneumonia is often said to be less serious than usual pneumonia because the symptoms that people will experience are milder as compared to regular pneumonia.
It is best to have yourself checked immediately when you start to experience some symptoms so that you can get the right treatment immediately.