There are many pneumonia causes and risk factors that can facilitate the development of the disease. Pneumonia is inflammation and infection of the lungs, generating symptoms like cough, headache, chest discomfort when breathing, difficulty breathing, shortness of breath, muscle pain, fever and a general state of fatigue. Although the symptoms of pneumonia may not appear to be serious, resembling those of flu or cold, some forms of the disease need immediate medical assistance and treatment.

Pneumonia can be developed by anyone, regardless of age and sex. Around 3 million people in the United States are diagnosed with pneumonia each year. Despite the fact that some forms of pneumonia can be very mild, other forms of the disease can actually be life-threatening. Statistics indicate that around 5 percent of hospitalized people with pneumonia die each year as a result of complication.

There are various pneumonia causes. Typical forms of pneumonia are developed due to infection with viruses (influenza, herpes simplex virus, varicella-zoster, adenovirus, respiratory syncytial virus), gram-positive bacteria (Streptococcus pneumoniae, Staphylococcus aureus, Streptococcus pyogenes) and gram-negative bacteria (Haemophilus influenzae, Klebsiella pneumoniae, Neisseria meningitides, Pseudomonas aeruginosa). Atypical pneumonia causes are numerous. Walking pneumonia is a very common form of atypical pneumonia and it is caused by infection with Mycoplasma pneumoniae. Mycoplasmas are microorganisms that share the characteristics of both viruses and bacteria, and these infectious agents can sometimes cause severe forms of pneumonia. Another pneumonia cause is infection with microorganisms such as Chlamydia pneumoniae and Legionella pneumophila.

When pneumonia is caused by viruses, the disease is usually less serious and in many cases the infection clears on itself. However, if pneumonia is caused by bacteria, the disease can become very serious. Sometimes, even neutral microorganisms can be a pneumonia cause. On the background of general body weakness, prolonged medical treatments, previous severe infections, immune system dysfunctions, recent chemotherapy or surgical interventions, some otherwise benign microorganisms can lead to the development of pneumonia. Under special circumstances, infection with neutral microorganisms can be a pneumonia cause. Infection with Pneumocystis carinii is considered to be the main Opportunistic pneumonia cause. Another common Opportunistic pneumonia cause is exposure to fungi, such as Mycobacterium avium.

Although the respiratory system is packed with natural defenses (mucus, nostril hairs, cilia), some infectious agents manage to break through these barriers and reach inside the organism. If microorganisms reach to the lungs, they quickly spread inside the alveoli (small pulmonary sacs of air), causing serious infections. Smokers and people with chronic obstructive pulmonary diseases are very exposed to contracting the infectious agents responsible for causing pneumonia.

Pneumonia isn't always caused by infection with microorganisms. Airborne irritants (particles of dust), exposure to chemicals, or inhalation of certain fluids can also become pneumonia causes. When pneumonia is developed due to inhalation of irritant matters, the disease is referred to as Aspiration pneumonia. This type of pneumonia is common in very small children and elderly people and it is generally a milder form of illness.