Healthy Living

What Pathogens Can Cause Pneumonia?


What are pathogens?

Viruses, bacteria, fungus, or, in some rare cases, parasites and other microorganisms are involved in the development of pneumonia. Viruses cause the majority of pneumonia in young children. Adults who have weaker immunity are more frequently infected with bacteria.

Pathogens, in simple words, are microorganisms that damage its host. They inherit a property that causes this damage. For example microbes like candida spp and staphylococci are already present in the individual but harm only a few. The same applies to other microbes to which individuals are either immune or have got vaccination.

The immune system does not differentiate between pathogens and non-pathogens. It reacts in a different way that completely depends on the characteristic of the host and the microbe. The response defines the outcome, which eventually reflects the behavior. Thus one can say that pathogenicity is due to low immune response to the pathogens rather than the pathogen itself.

There are three types of pathogens involved in causing pneumonia:

  1. Common pathogens
  2. Atypical pathogens
  3. Drug Resistant pathogens

Most common pathogens:

Streptococcus pneumoniae (Pneumococcus)
Mycoplasma pneumoniae
Chlamydophila pneumoniae

Respiratory viruses:

Hemophilus influenzae,
Legionella pneumophila
Moraxella catarrhalis

- Streptococcus pneumoniae, or pneumococcus, is the common pathogen that causes community-acquired pneumonia.

- Mycoplasma, Chlamydophila, and Legionella are the "atypical" CAP pathogens, causing up to one-third of CAP. Legionella causes severe CAP while mycoplasma and chlamydophila both cause mild disease.

- Influenza respiratory virus is the common reason behind CAP. Influenza infection predisposes the individual to subsequent progress of bacterial CAP called "post-influenza pneumonia."

- Hemophilus and moraxella are common pathogens in patients having a chronic obstructive pulmonary disease or intrinsic lung disease.

The "Atypical" CAP pathogens

Mycoplasma pneumoniae
Chlamydophila pneumoniae
Legionella pneumophilia

The above bacteria are responsible for more than 30% CAP among people. They are not clearly visible on sputum Gram stain analysis. These atypical pathogens cannot be killed by penicillin or other beta-lactam antibiotics.

Drug-resistant pneumococcus:

Drug-resistant streptococcus pneumoniae (DRSP) has become more commonly identified as a cause of CAP.  

Resistance to beta-lactam antibiotics

Drug-resistant streptococcus pneumoniae or DRSP have developed resistance to beta-lactam antibiotics, penicillins, and cephalosporin antibiotics by inhibition of antibiotic binding of the beta-lactam to bacterial enzymes involved in bacterial peptidoglycan synthesis. 

Resistance to macrolide antibiotics

Resistance to macrolide antibiotics is developed by 2 separate mechanisms: Mef (A)-mediated resistance involves an efflux pump, resulting in low-level resistance. Erm (B) mediated resistance involves a conformational change to the macrolide binding site at the bacterial 23S ribosomal subunit.  This change confers high-level macrolide resistance.  

Less Common CAP Pathogens:

Enteric Gram negative bacilli (Klebsiella, E. coli)
Pseudomonas aeruginosa
Group A Streptococcus
Endemic fungi
Staphylococcus aureus

Pneumonia-causing viruses

Over one-third of pneumonia is caused by viruses, and it usually affects children below 5 years of age.

Respiratory syncytial virus (RSV):

This respiratory virus is the major reason for pneumonia and bronchiolitis infection in children.

Human rhinovirus:

This virus is one of the reasons for lower respiratory tract infection, followed by common cold, and affects people with less immunity.

Herpes Simplex Virus (HSV):

This virus can affect people with good or weak immune system.


Pneumonia-causing fungus

The fungal infections that can lead to pneumonia usually originate from the soil. When the immune system gets weak, fungus in the body starts attacking which then leads to infection.


This fungus is more commonly found in warm and arid regions. Infection happens after the spores of fungus get inhaled.


This fungus is found in droppings of birds and bats in humid areas. This infection gets fatal if it spreads beyond the lungs.


It is usually found in soil contaminated with bird droppings and grows as yeast. They enter the lungs when the fungus-contaminated air is inhaled.

Transmission of pathogens

Pathogens like campylobacter that causes diarrhea, clostridium boltulinum, listeriosis, norovirus, salmonella and shigella are mostly transmitted by water or food.

Airborne pathogens are different and usually irritate the nose and throat, and they pass to other parts of the body through saliva and mucus. Airborne pathogens are the most dangerous. They include rhinovirus and corona virus.

Pathogens are disease-causing microorganisms. The body itself has several defense mechanisms to prevent pathogens to enter to the body. The immune system is designed to destroy pathogens. The diseases caused by pathogens are mostly because of the body's low immunity to pathogens rather than the pathogens itself.

Getting infected with pathogens does not necessarily lead to a disease. Infection occurs when the microorganisms multiply and disease when occur in the infected people when the cells of the body are damaged and symptoms of illness start appearing. To respond to infection the immune system puts white blood cells, antibodies fight with the invader. Headache, fever, rash, and other forms of malaise are a few symptoms that result when the immune system fights to eliminate infection from the body.


Antibiotics are used to kill the bacteria but not the viruses. There are a few antibiotics used to treat fungal infections. The medicine works two ways: they either kill the bacteria or stop them from reproducing. But the growing antibiotic resistance will curb the effectiveness of the drugs. Antibiotics should be taken exactly as directed by the doctor. Usually, the patient should continue taking it even after the symptoms subside to prevent the development of resistant bacteria.

Antiviral drugs are prescribed for viral infections. They fight the infection either by inhibiting the ability of the virus to reproduce or they strengthen the immune system.

Immunizing against pathogens is possible with vaccination. Over time, it is possible that bacteria becomes resistant to some antibiotics. MRSA is a methicillin resistant staphylococcus aurous. It resists most of the antibiotics. In order to prevent this, it is recommended to avoid the unnecessary use of antibiotics and, if taken, to always take a complete course.

In vaccination, very little portion of inactive, harmless pathogen enters the body to produce toxins that will fight the active pathogen. In effect, they act like antigens. So even if the person is affected later, the lymphocytes will reproduce and destroy the microorganisms.

Key Takeaways

  • Definition of pathogens
  • Kinds of pathogens that cause pneumonia
  • Transmission of pathogens
  • Treatment