Pneumonia is the inflammation in the lungs that is primarily caused by infections due to micro organisms like bacteria, virus and fungi. Additionally, the pneumonia can be caused by parasites and idiopathic reasons.

The bacteria and viruses are the most common infectious agent followed by fungi and parasites. In some cases, the mixed infections involving both viruses and bacteria may occur.

How infectious agents are transmitted to body?

The infectious agents of Pneumonia such as bacteria, viruses, fungi, parasites, etc; are transmitted to human body through various ways.

                    There is presence of Bacteria and viruses in our nose, sinuses, and mouth and these can get spread to the lungs in certain circumstances.

                    The germs are present in the air and these can get through the immune system present in our respiratory system to reach directly into the lungs during inhalation.

                    The germs can also be transferred through food or fluids from the mouth into the lungs.

Let’s discuss briefly about common infectious agents of Pneumonia.


Bacteria are the most common cause of Pneumonia. They normally affect the adult population.

The different strains of bacteria are associated with various risk factors.

                    Alcoholism: Streptococcus pneumoniae, anaerobic organisms, and Mycobacterium tuberculosis.

                    Smoking: Streptococcus pneumoniae, Haemophilus influenzae, Moraxella catarrhalis, and Legionella pneumophila

                    Exposure to birds: Chlamydia psittaci

                    Exposure to farm animals: Coxiella burnetti

                    Aspiration of stomach contents: Anaerobes

                    Cystic fibrosis: Pseudomonas aeruginosa and Staphylococcus aureus


After Bacteria, Viruses account for majority of the cases of Pneumonia especially in adults. As per the estimates, viruses are responsible for one third of Pneumonia cases.

Common Viruses that cause Pneumonia are as follows:

                    Rhinoviruses, Coronaviruses, Influenza virus

                    Respiratory syncytial virus (RSV), Adenovirus

                    Parainfluenza, Herpes simplex virus (causes Pneumonia in newborns)

                    Cytomegalovirus (CMV) (Causes Pneumonia in patients with weakened immune systems)


The pneumonia caused by fungi may occur in persons that have weakened immune systems. The fungal pneumonia has similar pathophysiology as that of bacterial pneumonia.

The immune system of the individuals might get weak due to various reasons including immunosuppressive drugs, AIDS or any other medical diseases. Thus, they become susceptible to fungal infection leading to Pneumonia.

Common Fungi that cause Pneumonia are as follows:

                    Histoplasma capsulatum, blastomyces

                    Cryptococcus neoformans

                    Pneumocystis jiroveci

                    Coccidioides immitis


The parasites infect the human body through the skin or the mouth. They reach the lungs via bloodstream; cause cell damage and triggers immune response leading to pneumonia. The response of eosinophils to the parasite infection in the lungs results in eosinophilic pneumonia.

Common parasites that cause Pneumonia are as follows:

                    Toxoplasma gondii

                    Strongyloides stercoralis


Idiopathic interstitial pneumonia

Apart from infectious pneumonia causes by Bacteria, Fungi, Virus and Parasites; there are certain causes of pneumonia that are idiopathic in nature and belongs to diffuse lung diseases.

Common lung diseases that cause Idiopathic interstitial pneumonia are as follows:

                    diffuse alveolar damage, lymphocytic interstitial pneumonia

                    organizing pneumonia, desquamative interstitial pneumonia

                    nonspecific interstitial pneumonia, interstitial pneumonia

                    respiratory bronchiolitis interstitial lung disease