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 and 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 then is exhaled out of the body, now in addition to air you’re constantly breathing in other stuff like microbes but we’re usually pretty 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 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 as well as 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 hemophilus 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 in the southwest which you can remember because there’s a c in both cocci and california histoplasmosis in ohio and the mississippi river valleys ancient histo in ancient ohio and blastomycosis which are broad-based budding yeast in the east, you remember that with east and yeast and the broad-based budding refers to the fact that under a microscope, when the fungi butt off each other there’s a broad versus a narrow base to round out the fungal causes in the U.S, there’s cryptococcus which is cryptic because geographically can pop up really anywhere, now one special fungal culprit is pneumocystis giuvechi which is a risk for immunocompromised individuals and 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 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.

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