How do you get pneumonia?

When you or your loved one is diagnosed with pneumonia, it is natural to wonder how you get pneumonia. Pneumonia is a common illness that can make you very sick. In my medical practice, I admit several patients to the hospital for the treatment of pneumonia. Most of my patients and their families ask me how one gets pneumonia. I am writing this detailed article to explain the many different factors that play a role in how and why you get pneumonia.

This article is based on my experience of treating pneumonia patients for the last 15 years as well as a thorough review of relevant medical literature.

In this article, we will focus on the most common type of pneumonia: bacterial community-acquired pneumonia. If you would like to read about different types of pneumonia and their symptoms, read this article.

Here is the sequence of events that play a role in why and how you get pneumonia:

Step 1: The bacteria responsible for causing pneumonia get inside your airway
Step 2: Bacteria overcome your airway defenses and go deep inside your lungs
Step 3: Bacteria survive attacks by special immune cells inside the air sacs
Step 4: Bacteria overwhelm your immune system and multiply inside your lungs
Step 5: Your lungs get filled up with pus and gunk as a result of the battle between your white blood cells and bacteria. You get diagnosed with pneumonia.

We will examine each step in further detail to properly understand why and how you get pneumonia.

Want to learn how you get pneumonia? Discover the five steps involved.

Step 1: The bacteria responsible for causing pneumonia get inside your airway

How do bacteria responsible for causing pneumonia get inside your airway?

The short answer is: You breathe in droplets containing bacteria.

The bacteria that causes community-acquired bacterial pneumonia is very common in the community. Most people who inhale droplets containing this bacteria do not get pneumonia. The droplets simply settle down inside the nose and upper airway. The bacteria colonize the upper airway. Your immune system then attacks these bacteria. After that, your immune system either gets rid of them or prevents them from invading any further. If your immune system doesn’t completely eliminate them, instead restricting them to your upper airway, you become a carrier of bacteria.

Where and how do droplets containing bacteria get into the air you breathe?

Bacteria-containing droplets are produced when someone carrying the bacteria breathes, coughs, sneezes, speaks, or sings into the air that you breathe. Unlike the coronavirus that causes COVID-19 pneumonia, bacteria that cause community-acquired pneumonia are everywhere in the community. There are many carriers of community-acquired pneumonia that are not sick. The exact percentages of people in any population that carry common pneumonia-causing bacteria vary significantly, but based on different studies, it may range from 5% to 20% of the population.

People that are sick from pneumonia represent only a small fraction of people that can transmit pneumonia-causing bacteria. That is why we don’t recommend isolating people with community-acquired bacterial pneumonia. Avoiding exposure to pneumonia-causing bacteria is very difficult, and it is not an effective way to prevent pneumonia.

Being exposed to pneumonia-causing bacteria is only the first step towards getting pneumonia. Most people that inhale droplets with bacteria don’t get sick. Since it is not practical to avoid inhaling droplets with pneumonia-causing bacteria, the prevention of pneumonia is focused on other steps necessary to get pneumonia.

Step 2: Bacteria overcome your airway defenses and go deep inside your lungs

How does your upper airway defend against pneumonia?

Your nose is your first line of defense against pneumonia. Nasal hair traps larger droplets and filters the air. Besides the visible hair, the inside of your nose also has small microscopic hair-like objects called cilia. These cilia are constantly moving in a sweeping manner. They trap bacteria and sweep them away.

Cilia are present in your airway all the way from your nose to deep inside your lungs. Anything that affects cilia and their ability to sweep away bacteria makes it easier for bacteria to invade deeper into your lungs, causing pneumonia.

How do pneumonia-causing bacteria overcome cilia and cause pneumonia?

The most common cause of damaged cilia is smoking cigarettes. Multiple studies have proved that smokers are significantly more likely to get pneumonia compared to non-smokers. Damage to cilia is one of the ways smoking makes it easier to get pneumonia.

A viral infection, such as influenza, can also damage the lining of your airway and make it easier for pneumonia-causing bacteria to invade deeper inside your lungs. You can read the article “Pneumonia after the flu” if you would like to learn more about how your risk of getting bacterial pneumonia goes up after the flu.

Air pollution is another cause of cilia damage. People who live in places with poor air quality are at increased risks of getting pneumonia.

People with pre-existing lung problems such as COPD, asthma, or lung cancer also have cilia that don’t function properly. These people are are more likely to get pneumonia.

Step 3: Bacteria survive attacks by special immune cells inside the air sacs

What happens when bacteria reach the air sacs?

Once bacteria reach the deep air sacs, they face their first battle with your immune system. Your air sacs are guarded by immune cells called macrophages. Macrophages are immune cells that eat up anything harmful that makes it to the air sacs. After eating bacteria, macrophages kill them inside the cells. Killing of bacteria takes time, and macrophages can’t eat any new bacteria until the majority of eaten-up bacteria have been destroyed and broken down.

When the load of bacteria reaching the air sacs is too much, macrophages can’t keep up with bacteria. When that happens, bacteria roam freely inside air sacs, getting ready to grow and multiply. However, this is still too early to get pneumonia. Your immune system can still ward off bacteria, and you might not actually get pneumonia.

There are certain things that can weaken your macrophages and make you more susceptible to getting pneumonia. Alcohol abuse is the most common one. Multiple studies have identified alcoholism as a cause of significant macrophage dysfunction. Alcoholics are at a significantly higher risk of getting pneumonia.

Poor nutrition is another cause of macrophage dysfunction. It is less common in the United States, but happens frequently in developing countries. A lack of zinc in the diet has been specifically linked to dysfunctional macrophages.

Step 4: Bacteria overwhelm your immune system and multiply inside your lungs

When macrophages inside the air sacs get overwhelmed trying to fight bacteria, they call for reinforcements. Additional immune cells called neutrophils arrive to help eat up and kill more bacteria. Macrophages also produce chemicals that help regulate other types of immune cells. The goal is to neutralize the invasion without damaging lung tissue. In many people with healthy immune systems, this goal is achieved. Many healthy people do not get pneumonia even when pneumonia-causing bacteria successfully reach the air sacs in large enough numbers to overwhelm macrophages.

After some time, the immune system starts to recognize specific proteins of the bacteria and produces specific antibodies. These antibodies recognize and bind to the proteins, activating chemicals that destroy targeted bacteria. People who have been vaccinated with pneumonia vaccines produce these antibodies earlier and more effectively. If you have been vaccinated, you have antibodies in your system, making it easier to recognize bacteria and produce more antibodies to kill them. This is especially important in people who have weaker immune systems, such as the elderly and people with chronic illnesses.

Step 5: Your lungs get filled up with pus and gunk due to the battle between white blood cells and bacteria, leading to pneumonia.

Unfortunately, the immune system is not always able to get rid of bacteria. When bacteria continue to grow inside your lungs, your immune system becomes overwhelmed. It starts to produce too many inflammatory chemicals. These chemicals help kill bacteria, but they also damage the lungs.

When you reach this stage, you get pneumonia. Large numbers of white blood cells accumulate inside your lungs to continue the fight with bacteria. Lots of fluids accompany white blood cells, and affected air sacs get filled up with a pus-like fluid. At this point, these fluid-containing air sacs can be seen as white areas on a chest x-ray. You are now diagnosed with pneumonia.

This is how you get pneumonia. It takes several steps after inhaling pneumonia-causing bacteria. It is difficult to avoid inhaling bacteria because they are present everywhere; instead, these steps help prevent pneumonia:

  1. Stop smoking
  2. Avoid getting the flu
  3. Avoid excessive alcohol use
  4. Get vaccinated against pneumonia if you are at a high risk
  5. Take care of your chronic medical issues

In conclusion, inhaling pneumonia-causing bacteria doesn’t necessarily mean getting pneumonia. It takes several steps to get pneumonia, and it is more effective to address preventative measures in the subsequent stages.