Pneumonia is a complex condition to classify. It is normally the result of other infections rather than being a disease in its own right. Pneumonia can also be caused by many microorganisms. Most physicians identify three main types of pneumonia: bacterial, viral and mycoplasma. Mycoplasma are tiny organisms that behave similarly to bacteria and viruses. However, there are many more causes of pneumonia, including some fungi and chemical agents.

Bacterial pneumonia is caused by bacteria infecting the lungs. Most people normally have potentially dangerous bacteria in their nose and throat, but they rarely infect the lungs. However, when the immune system is compromised, these bacteria have the opportunity to multiply and cause an infection. Bacterial pneumonia can occur while the body is recovering from influenza (flu). In the United States, bacterial pneumonia is most often caused by the bacteria Streptococcus pneumoniae. Streptococcal pneumonia can easily spread to other parts of the body where it can cause further problems such as meningitis, bacteremia (bacteria in the blood) and otitis media (middle ear infection). Bacterial pneumonia can usually be effectively treated with antibiotics.

Viral pneumonia accounts for up to half of pneumonia cases in the United States. It is commonly caused by an influenza virus. Therefore, most cases of viral pneumonia are reported in the winter, when influenza viruses are predominant. People with viral pneumonia are also at a high risk of developing bacterial pneumonia. Viral pneumonia is more serious in people with chronic illnesses such as cardiovascular or lung disease, as well as pregnant women. Because antibiotics cannot treat viruses, there are limited treatment options available for viral pneumonia. Most cases clear up over a certain period of time with supportive care.

Mycoplasma pneumonia is sometimes called walking pneumonia because its symptoms are often mild and may not require bed rest. The lungs are invaded by tiny organisms called mycoplasms. These were initially thought to be viruses, but are now understood as being neither bacteria nor viruses, but organisms that behave a little like both. Mycoplasma pneumonia spreads easily in places where people have close contact, such as day care centers and schools. Studies have suggested that mycoplasma pneumonia could account for up to one-third of all childhood cases of pneumonia.  

In addition to the three main types of pneumonia, there are many other types that may affect portions of the population. These include:

• Fungal pneumonia. Various types of fungi can cause pneumonia. Some of these fungi are only found in certain parts of the United States. For example, the Histoplasma capsulatum fungus is only found in the Mississippi River and Ohio River valleys and Coccidioides immitis fungi only occur in the semiarid climate of the southwestern United States. Symptoms can vary in cases of fungal pneumonia but are often only mild. Some types of fungal pneumonia produce no symptoms at all. Fungal pneumonia is often more serious in people with impaired immune systems.
  
  
• Pneumocystis jiroveci pneumonia (PCP). Previously called pneumocystis carinii pneumonia. This type of pneumonia is particularly dangerous for people with HIV or AIDS. It is caused by a fungus that is not normally harmful for healthy people but can cause lung infections in people with severely compromised immune systems. Often, the first sign that someone has developed AIDS is the diagnosis of PCP. PCP can be prevented using antibiotics.
  
  
• Chlamydia pneumonia. This type of pneumonia is most common in school-aged children and has mild symptoms, similar to mycoplasma pneumonia. It can normally be effectively treated with antibiotics.