Many microorganisms can cause pneumonia. The most common are bacteria and viruses that are present in the air, we breathe. Fungi can also cause pneumonia, though fungal pneumonia is not so common.
Our body’s immunity overcomes these microorganisms that we breathe in, and prevents them from infecting our lungs. But, when the virility of these microorganisms is strong or when the body’s immunity becomes weak, these microorganisms can overpower your immune system, and cause a respiratory infection.
Pneumonia is a pulmonary infection and can be caused by bacteria, viruses, or fungi. Let us discuss each causative agent in detail.
Bacterial Pneumonia
Bacterial pneumonia can be caused by various bacteria. The most common type of bacterial pneumonia is called pneumococcal pneumonia, which is caused by the Streptococcuspneumoniae bacteria and isolated in nearly 50% of bacterial pneumonia cases. Pneumococcal pneumonia kills about 1 in 20 people who get it.
Streptococcuspneumoniae bacteria normally reside in the upper respiratory tract without causing any harm but becomes pathogenic when the immune system is suppressed.
Bacterial infection of bacterial pneumonia develops in the alveoli (air sacs) of the lungs. It can develop due to direct bacterial infection or as a super-added bacterial infection after a person has had an active viral infection such as a cold or flu.
Bacterial pneumonia is often called lobar pneumonia, because it affects just one part or lobe, of a lung.
Bacteria that cause pneumonia can infect your lungs when you inhale respiratory droplets, which an infected person releases into the air when he coughs, sneezes, or talks.
You can also get pneumonia by touching an object that has these pneumonia-causing bacteria on it and then touching your nose or mouth.
Streptococcus pneumoniae is a gram-positive bacteria that is a member of the Streptococcus family and is the main cause of community-acquired bacterial pneumonia (CAP).
S. pneumoniae normally always resides in the throat and nasal cavity of healthy individuals and does not cause any disease.
However, this bacteria can spread to the lungs and cause pneumonia infection in people who have suppressed or under-developed immune systems.
Such individuals include infants, the elderly over the age of 65 years, people who are suffering from a chronic pulmonary disease, or those on chemotherapy. A vaccine against S. pneumoniae infection is strongly recommended especially for patients belonging to this high-risk group.
Other bacteria that cause bacterial pneumonia include Legionella pneumophila, Mycoplasma pneumoiae, and Chlamydophila pneumonia. They cause a milder form of pneumonia called atypical pneumonia or walking pneumonia, where the person need not be in bed but could be walking around.
They are referred to as atypical because their symptoms are different,, their findings on a chest X-ray are different, and they respond to different antibiotics than the typical antibiotics that are used for typical pneumonia. Being called “atypical,” does not mean they are rare; they are fairly common.
Patients with pneumonia due to Legionella pneumophila are typically older (over 50 years), chronic smokers, and with a weakened immune system. Unlike other bacterial cases of pneumonia, Legionella does not spread from person to person. This bacteria grows in man-made water habitats including whirlpool spas, outdoor fountains, water tubs, plumbing systems, and cooling towers. Outbreaks of Legionella have been linked to exposure to such contaminated water. It can lead to a serious type of pneumonia known as Legionnaires’ disease, which can be fatal if left untreated. However, otherwise, most people recover.
Mycoplasma pneumonia is typically common in crowded living spaces like dormitories. It affects younger adults (under age 40), and symptoms are typically mild.
Pneumonia due to bacterium Chlamydia pneumonia is usually mild pneumonia, and most often seen in people more than 40 years old.
People at a higher risk of contracting bacterial pneumonia include people recovering from surgery, people with chronic pulmonary disease, those with an active respiratory viral infection, those who smoke or drink too much alcohol, those who have diabetes or hypertension, and those people with a weakened immune system.
Pneumonia caused by viruses
Viral pneumonia is the most common, with more prevalence than bacterial or fungal pneumonia.
It is especially most common in children. Common cold viruses such as rhinovirus and the influenza virus cause upper respiratory tract infections that can spread to the lungs and cause pneumonia.
Viral pneumonia is mild in severity and shorter in duration than bacterial pneumonia.
However, sometimes viral pneumonia, especially that caused by the influenza or flu virus can become quite severe, especially in pregnant women or in those who suffer from heart or lung conditions such as cor pulmonale.
Viral pneumonia can sometimes develop a super-added bacterial infection in which case viral and bacterial pneumonia can co-exist.
Viruses that can cause viral pneumonia include:
Influenza (flu) A and B viruses are the most common cause of viral pneumonia in adults
Respiratory syncytial virus (RSV), is a virus that causes infection of the respiratory tract. It is the most common cause of pneumonia in children younger than 1 year of age.
Coronaviruses, including SARS-CoV-2, the virus that causes COVID-19
Rhinoviruses (causes common cold), parainfluenza viruses (cause upper and lower respiratory illnesses), and adenoviruses (that cause upper and lower respiratory tract infection and eye infection)
Other viruses that are rare causes of viral pneumonia include herpes simplex, measles, and chickenpox viruses.
The mode of transmission of the viruses that cause pneumonia is mainly through the air.
When an infected person sneezes or coughs or even talks, he releases virus-containing droplets into the air which if inhaled or swallowed by a healthy individual, can infect him.
You can also get viral pneumonia after touching a virus-covered object or surface such as a doorknob and then touching your mouth or nose.
Pneumonia caused by fungi
Fungi are microorganisms that move around by spreading out or sending spores (microscopic cells) into the air or environment.
Fungi cause fungal pneumonia. Though fungal pneumonia is not common, it is mostly seen in individuals with compromised immunity such as those with HIV infection, or those with chronic diseases such as diabetes, hypertension, heart disease, cancer, COPD, and asthma.
Mortality in fungal pneumonia can be as high as 90% in such immunocompromised patients.
Some types of endemic fungi that cause fungal pneumonia include coccidioidomycosis, histoplasmosis, and blastomycosis.
Fungal pneumonia is not contagious, meaning it does not spread from person to person. You usually contract fungal pneumonia by inhaling air-borne fungal spores from the environment.
Fungal spores exist in soil, plants, and animal droppings and can also contribute to its spread through this source. When the contaminated soil or animal droppings are disturbed, tiny fungal spores are released into the air, which can be inhaled.
This most commonly occurs during activities like construction, gardening, or cleaning up areas with large accumulations of bird or bat droppings.
These inhaled fungal spores then get lodged in the lungs and cause an infection. This is the most common way that you can catch fungal pneumonia.
Fungal pneumonia can also be caused by reactivation of a latent infection. The fungus that has been dormant in the body can become active again when the immune system is weakened.
Risk Factors
There are several risk factors that make you prone to developing pneumonia. They include:
Age. Children below two years old and the elderly over 65 years are more likely to get pneumonia.
Weakened immunity. A weakened or underdeveloped immune system increases the risk of pneumonia. Immunity can be compromised by health conditions like HIV/AIDS, organ or bone marrow transplants, chemotherapy, long-term use of steroids, and pregnancy.
Chronic lung diseases. You’re more likely to get pneumonia if you have asthma, chronic obstructive pulmonary disease (COPD), bronchiectasis, or cystic fibrosis.
Brain disorders, such as a stroke, a head injury, dementia, or Parkinson’s disease can cause difficulty in swallowing, which can lead to aspiration of drink, vomit, or saliva. This can increase your risk of aspiration pneumonia.
Other serious conditions, such as malnutrition, diabetes, heart disease, liver or kidney disease, and sickle cell disease are additional risk factors.
Lifestyle. Certain lifestyle habits increase your risk. They include regular smoking or chronic exposure to secondhand smoke, and alcohol abuse. Poor dental hygiene also increases your risk by allowing the bacteria to proliferate in the mouth, which can then travel or be inhaled into the lungs and cause a lung infection.
Environment. Regular exposure to air pollution and toxic gas can increase the risk of pneumonia.
Living areas. Living in enclosed places where cross-infection is possible like a general ward or ICU of a hospital or nursing home, prison, military barracks, or a crowded shelter, can also increase the risk. Being sedated or unconscious can increase the risk of aspiration pneumonia. Being on a ventilator in the hospital can increase your risk hospital hospital-acquired pneumonia (HAP) and ventilator-acquired pneumonia (VAP).
Recent illness. Having recently recovered from an acute attack of a common cold or the flu can increase the risk of pneumonia.
