When we think of the deadliest diseases of the world, things like Ebola and the Black Death come to our minds. However, it turns out that cardiovascular disease (CVD) is the world’s leading cause of death in this day and age. In the 1970s, we were convinced that CVD was caused by dietary fat. Indeed, there was a public campaign to stamp out fat and replace it with carbohydrates. People didn’t realize that fat wasn’t the issue. Rather, fat has the highest calorie/mass ratio (9 calories per gram) and excess consumption of fat leads to obesity and its milieu of problems, including CVD. The food industry replaced fat with carbohydrates to meet this craze, and “fat-free” foods were instead pumped with sugar. Of course, calories are calories—and when people consumed these foods in earnest, not much changed with regards to the incidence of CVD.

So what exactly is fat? Chemically, fat is a glycerol molecule + three fatty acids. Fatty acids are long-chained aliphatic carboxylic acids that are either saturated (only single bonds) or unsaturated (one or more double/triple bonds). Quick aside: in popular culture, we hear about “omega-3 fatty acids” and “omega-6 fatty acids.” This refers to the position of the first double bond in the fatty acid, with the omega position being the last carbon in the chain (see picture below). So an omega-3 fatty acid is a fatty acid that has its first double bond on the 3rd carbon from the omega position.

Origin: Greek, glykeros (sweet) + –ol (chemical suffix for alcohols)
So named because glycerol is, in fact, sweet (and is an alcohol)

triglyceride glycerol

Origin: Greek, aleiphar (oil)
Refers to hydrocarbons that do not contain benzene rings

What about the term lipid [Gk. lipos (fat)]? In the vernacular, people equate the word “lipid” to “fat,” and its etymology certainly vindicates such an equation. But of course, we must be rigorous with our definitions. To start, fats (i.e., triglycerides) are certainly a lipid subclass. Other subclasses include waxes (fatty acid esters such as beeswax; article’s featured image) and sterols (isoprene-derived steroids such as cholesterol). One classical definition of a lipid is the following: lipids are hydrophobic (some also include amphiphilic) small molecules that can dissolve in organic solvents but not water.

Origin: Greek, hydor (water) + phobos (fear)
Fear of water, i.e., not dissolvable in water or other polar solvents

Origin: Greek, amphi – (both) + philia (love)
Love of both water and fat, i.e., dissolvable in polar and nonpolar solvents

For those who love math, recall that for a statement to be proven true, it must be true in all instances. For a statement to be proven false, one need only provide a counterexample. So my counterexample for this definition of lipid is the molecule carbon tetrachloride (CCl4). CCl4 is a small molecule, it is hydrophobic, it dissolves in organic solvents, and it doesn’t dissolve in water. Now, to patch the definition up, one can make an addendum to include “naturally-occurring.” Needless to say, CCl4 is not naturally-occurring. Is there another definition of lipid that you prefer? Let me know in the comments!


Before ending this post, I would like to touch on the significance of fat in disease. There are a number of disease states that manifest with malabsorption; one of them is coeliac disease. The diarrhea that coeliac patients experience after ingesting gluten is more accurately termed steatorrhea:

Origin: Greek, stear (fat) + rhein (to flow)
Flow of fat (through the intestines), i.e., fat in the stools

How can you identify steatorrhea? If the stool is especially foul-smelling and sticks to the sides of the porcelain shrine, you’ve got yourself a flow of fat.

Pneumonia is one of those medical conditions that is constantly thrown around without being rigorously defined—and as we all know by now, rigor is absolutely paramount in my view of academic medicine and medical etymology. Let’s start by looking at the word from an etymological standpoint:

Origin: Greek, pneumon (lung) + –ia (condition of)
Condition of the lung

Well, that certainly wasn’t very helpful—at least not at first glance. There can be many conditions of the lung that are called “pneumonia” and the classic case of a lung infection is merely one of them. Compare pneumonia to pneumonitis:

Origin: Greek, pneumon (lung) + -itis (inflammation of)
Inflammation of the lung

There are many inflammatory conditions of the lung, but only a few of them are called “pneumonitis” because pneumonitis is considered to be inflammation of the lung tissue itself (the lung parenchyma). There are some who then consider pneumonia to be inflammation of the air sacs in the lungs (the alveoli). This makes sense for the most part; the bacteria, viruses, fungi, and parasites that can cause pneumonia would cause alveolar inflammation.

lung diagram

To put things in perspective, when somebody is admitted to the hospital for pneumonia, the classic picture is someone who acquires a bug, and said bug colonizes the alveolar space. The resulting “mass” that forms (usually in one lobe of the lung) is called a consolidation:

Origin: Latin, con- (with) + solidus (solid)
With solid, i.e., things that aggregate to form a “solid”

For these “classic” pneumonias, we can call them community-acquired pneumonia (CAP), hospital-acquired pneumonia (HAP), and the like. Some bugs associated with the former are things like Streptococcus pneumonia and Pseudomonas aeruginosa, and some bugs associated with the latter are things like Staphylococcus aureus and Haemophilus influenzae. (Stay posted for microbiological etymology!)

But it turns out that this isn’t the whole picture. Pneumonia is indeed as general as its etymology suggests, for there can be inflammatory and fibrotic conditions of the lung—and both classes can fall under the term “pneumonia.” (Generally, inflammation is caused by the infiltration of white blood cells and fibrosis is caused by the production of excess connective tissue, e.g., collagen.) Thus, as a counterexample to the definition that pneumonia is simply the inflammation of the alveoli, we have something called usual interstitial pneumonia (UIP). It is a form of interstitial lung disease in which the tissue between the alveoli is fibrosed with collagen.

uip ctThe classic “honeycomb lung” appearance of UIP on CT scan

To end this post, I posit my generalized definition of pneumonia: pneumonia is a principally inflammatory or fibrotic condition of the lung. By formalizing this definition of pneumonia, I am faithful to the etymology and make pneumonitis a subset of pneumonia. Note that I use the word “principally” in front of inflammatory—the reason why is that cancers of the lung can cause inflammation, but cancers are not considered to be pneumonias by any definition or practice.

In another post I will discuss the classic methods of diagnosing “normal” (generally bacterial) pneumonia and the etymologies of these techniques. Here is a small teaser: one of the terms involves the Greek word for goat! Can you guess which one? Leave a comment below.