Pneumonia is an infection in the lung tissue caused by microbes, and the result is inflammation.
The inflammation brings water into the lung tissue, and that extra water can make it harder to breathe.
During inhalation, air reaches your lungs by traveling down your trachea, then it continues through the bronchi and the bronchioles and ends up in the alveoli.
The alveoli are tiny air sacs that look like tiny clumps of grapes, that are wrapped up in a net of capillaries. This is where the majority of gas exchange happens in the lungs.
Oxygen leaves the air in the alveoli and crosses into the bloodstream while carbon dioxide leaves the bloodstream and is then exhaled out of the body.
Now, now in addition to air, you’re constantly breathing in other stuff, like microbes. But we’re usually good at protecting ourselves.
For example, we have mechanical techniques like coughing, a mucociliary escalator that lines the entire airway and moves out larger bacteria, and macrophages that are nestled deep inside the alveoli and ready to destroy anything that lands there.
But sometimes, a particularly nasty microbe might succeed in colonizing the bronchioles or alveoli, and when that happens - Congratulations! You’ve got pneumonia.
Those microbes typically multiply and cross over from the airways into the lung tissue, creating an inflammatory response.
The tissue quickly fills with white blood cells as well as proteins, fluid, and even red blood cells if a nearby capillary gets damaged in the process.
Now, there are lots of different pneumonia-causing microbes.
Usually it’s caused by viruses and bacteria, but it can also be caused by fungi and a special class of bacteria called mycobacteria.
In adults, the most common viral cause of pneumonia is influenza, sometimes just called the flu.
In adults, bacterial causes include streptococcus pneumoniae, haemophilus influenzae, and staphylococcus aureus.
There are also more unusual bacteria like mycoplasma pneumoniae, chlamydophila pneumoniae, and legionella pneumophila, which don’t have a cell wall and are well known for causing an “atypical or walking pneumonia” because they often cause vague symptoms like fatigue.
In individuals with a normal immune system, fungi are a rare cause of pneumonia and often it’s regional - for example, there’s Coccidioidomycosis in California and the Southwest - which you can remember because there’s a “C” in both cocci and california, Histoplasmosis in the Ohio and mississippi river valleys - “H” in Histo and in O”H”io, and Blastomycosis which are broad based budding yeast which are in the east - you can remember that with the “east” in yeast.
And the broad based budding refers to the fact that under a microscope, when the fungi bud off of each other there is a broad versus a narrow based.
To round out the fungal causes in the US, there’s Cryptococcus which is “cryptic” because geographically it can pop up really anywhere.
Now, one special fungal culprit is pneumocystis jiroveci which is a risk for immunocompromised individuals.
Finally, there’s mycobacteria which are slow-growing like fungi, hence the “myco” in their name even though they’re still bacteria.
The most well known one is mycobacterium tuberculosis, also just called TB.
Pneumonia can also be categorized by how it’s acquired.
The most common, is community acquired pneumonia, and it’s called that when a person gets sick outside of a hospital or healthcare setting.
Next is hospital-acquired pneumonia or nosocomial pneumonia, which is when a person gets pneumonia when they are already hospitalized for something else.
This type tends to be more serious because sick patients often have a weakened immune systems and the microbes in hospitals are often resistant to the common antibiotics. That’s because hospitals bring together the bacteria that are often the most virulent - think great offense - as well as the most resistant - think great defence.
These bacteria are able to swap some of the antibiotic resistance genes with one another.
A well known example is Methicillin-resistant staphylococcus aureus, or MRSA.
Non resistant staph aureus can cause pneumonia and other infections, but it can also be killed by common antibiotics like ampicillin.
MRSA on the other hand is resistant to many antibiotics and is therefore harder to treat.
Another category of pneumonia is ventilator associated pneumonia, which is a subset of the hospital-acquired pneumonia, but it specifically develops when ill individuals are connected to a ventilator.
Oftentimes, there’s a biofilm - which is a mix of bacteria and sugars and proteins that can coat a surface - that forms on the endotracheal tube.
Individuals on a ventilator can’t cough and are often quite sick already, so over time microbes can move from the tube directly into the lung and cause a pneumonia.
Now in addition to inhaling microbes there are other ways to develop pneumonia.