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What Is Atypical Pneumonia?

Pneumonia is an infection of the lungs by a virus, bacterium, or fungus. Despite its name, atypical pneumonia is a common bacterial infection that is considered to be “atypical” based on its nontraditional clinical presentation and the difficulty in detection through standard detection methods.

Even more, atypical pneumonias usually present as patchy infiltrates (white spots on the lungs) on X-rays as opposed to the lobar infiltrates (inflammation of the entire lobe) that are often seen in pneumonias caused by Streptococcus pneumoniae and other bacteria.1

Also known as walking pneumonia, atypical pneumonia is usually mild and follows a different clinical course of disease despite its ominous appearance on X-ray imaging. 

This article covers the causes, types, symptoms, and treatment of atypical pneumonia.


"Atypical pneumonia" is almost a misnomer because the same four bacteria typically cause this disease. The following bacteria are the most common causes of atypical pneumonia:

  • Chlamydia pneumoniae
  • Chlamydia psittaci
  • Legionella pneumophila
  • Mycoplasma pneumoniae

Chlamydia Pneumoniae

This highly transmissible bacterium that circulates via tiny air droplets, spreads quickly in crowded environments like:

  • Schools
  • College dormitories
  • Hospitals
  • Mass incarceration sites
  • Nursing homes
  • Cruise ships

Currently, there is no national surveillance system to track the disease caused by Chlamydia pneumoniae, but clinical data show that older adults exposed to Chlamydia pneumoniae are often at a greater risk of developing severe disease, underscoring the importance of isolation and quick initiation of treatment once the organism is identified in a population.

Some people will show no symptoms, while others may develop symptoms of:

  • Runny nose
  • Cough
  • Sore throat
  • Low fever

These symptoms typically present three to four weeks after exposure to the bacteria

Chlamydia Psittaci

Chlamydia psittaci is a bacterium that usually infects birds but may also infect humans who keep parrots and parakeets as pets.

Symptoms typically include:

  • Mild fever and chills
  • Muscle aches
  • Headache
  • Dry cough

Signs usually develop five to 14 days after infection and death is extremely rare.2

Legionella Pneumophila

Legionella pneumophila causes the most severe type of pneumonia, killing nearly 1 in 10 people.3 Infected older adults (ages 65 and up) and current or former smokers are at the greatest risk of severe outcomes.

If you have a chronic illness or a weakened immune system, you may also be at increased risk of catching the disease caused by Legionella and developing more severe symptoms.

The classic story that most medical students learn is the scenario in which Legionella spreads throughout a nursing home via the air-conditioning system. Legionella bacteria can multiply in freshwater environments, including systems made by humans.

Exposure to contaminated water sources or contaminated air droplets can cause disease, so it’s imperative that you know the danger that Legionella can pose in crowded settings. 

Signs will typically show two to 10 days after exposure to the bacteria.

Classic symptoms of Legionella exposure include:

  • Cough
  • Headaches
  • Muscle aches
  • Shortness of breath
  • Fever

Mycoplasma Pneumoniae

Mycoplasma pneumoniae causes the most common atypical and community-acquired form of pneumonia, infecting more than 2 million people per year.4

Like Legionella and Chlamydia pneumonia, Mycoplasma spreads from person to person by way of respiratory droplets and, therefore, thrives in crowded spaces. Outbreaks most often occur in:

  • Schools
  • Colleges
  • Hospitals
  • Nursing homes
  • Military barracks

Symptoms range from a mild chest cold and asthma-like shortness of breath to inflammation of the brain (encephalitis) and kidney problems if left untreated. 


The symptoms you develop largely depend on your general state of health and the type of organism that you have been infected with. Some symptoms that are common to all types of atypical pneumonia include:5

  • Fever (usually mild or moderate)
  • Chills
  • Cough (may be dry or productive)
  • Sore throat
  • Hoarseness or loss of voice
  • Headache
  • Chest pain when you breathe deeply
  • Shortness of breath
  • Fatigues
  • Sneezing

Usually, symptoms are mild in nature and resolve on their own over time. However, taking a full course of antibiotics as prescribed by a healthcare provider is recommended to shorten your course of infection.

It may take a month, or more, for your symptoms of atypical pneumonia to go completely away.


Taking antibiotics is important, not only because they decrease your symptom, but they kill the bacteria eliminating the spread of inflection.

If your healthcare provider thinks the cause of your symptoms is due to one of the four bacterias they may prescribe one the following antibiotics:1

  • Macrolide antibiotics: Macrolide drugs are the preferred treatment for children and adults. Zithromax (Azithromycin) and Biaxin (clarithromycin) are the two most prescribed macrolides, although there is growing concern over the rise of macrolide-resistance Mycoplasma pneumoniae over the past few decades. 
  • Fluoroquinolones: Cipro (Ciprofloxacin) and Levaquin (levofloxacin) may be effective in adults but should be avoided in children. 
  • Tetracyclines: Tetracyclines like doxycycline (Targadox) are especially useful when macrolides and fluoroquinolones are not effective or unavailable. They are suitable for both older children and adults.

When to See a Healthcare Provider

If you are not feeling well, and suspect that you have atypical pneumonia, do not take your symptoms lightly. The severity of atypical pneumonia can range from mild to life-threatening.

Although most cases can be resolved with outpatient antibiotics, some cases progress to:6

Seek medical attention if your condition does not change or worsens despite treatment

You may need to change your current antibiotic regimen or the need to get additional testing done to rule out other infections or the possibility that you have developed a medical complication. 


Atypical pneumonia is a common bacterial infection of the lungs. It is called “atypical” due to differences in its detection and clinical presentation compared to other forms of pneumonia.

A Word From Verywell

Atypical pneumonia is quite common, but it rarely leads to hospitalization. In fact, most people enjoy a full recovery in a few weeks. Taking your full course of antibiotics exactly as prescribed by your healthcare provider is key to eliminating and stopping the spread of disease. Bacterial resistance has been on the rise in recent years, rendering some of the most effective antibiotics useless, so it is vital to take your full course of antibiotics even if you no longer have symptoms.

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