We now know that pneumonia of any particular cause can have either typical or atypical features, so the terminology is not very helpful. They live only in cells and do not respond to antibiotics. Viral infections are difficult to diagnose in the early stages, in part because the organisms cannot be seen with an ordinary microscope. Immune depression may occur either from an underlying disease or from treatment for the disease. Examples include kidney transplant patients who are receiving immunosuppressive agents to help prevent rejection of the kidney, cancer patients receiving drugs that depress the immune system while attacking cancer cells, and patients with blood disorders such as multiple myeloma that directly affect the body's immune system.
Doctors generally go through four steps to make the diagnosis.
As in any disease, the diagnosis must first be suspected before it can be made. This is not usually difficult. In fact patients will often suspect the diagnosis even before consulting a physician, because of symptoms different from any flu or cold ever experienced. In step three the physician attempts to find the cause. This involves ordering certain tests to help determine which organism is responsible for the infection.