You're probably familiar with the long standing medical adage; "Inflammation is the root of all disease."
While this is perhaps a slight exaggeration, it is true that inflammation is a contributing factor to a huge number of diseases, injuries and other ailments of the body and brain.
Aside from inflammation's role in disease, living with inflammation on a day-to-day basis just sucks. An inflamed body feels achy and low-energy, looks puffy and certainly doesn't perform as well. An inflamed brain is slow and foggy and is at risk for more serious neurological disease.
Understanding what inflammation is, how it happens and what the symptoms are can be a huge asset in understanding why your body is feeling or responding a certain way at a certain time - and fixing the underlying issues.
What Is Inflammation?
Inflammation is the first line of defense our body uses to protect against infection. Fundamentally, it's a pretty simple mechanism:
When immune cells detect the presence of foreign microbes, they will send signals to the tissue cells in the surrounding area instructing them to increase blood flow to the area. Increased blood flow brings more immune cells (like macrophages) to combat the invading bacteria or fungus.
You know when you get a cut and the area around it gets red and puffy? That's your immune system and inflammatory response in action.
Sounds pretty useful, right? Yes, of course. Without our inflammatory response, every little paper cut would represent a significant risk for a system-wide bacterial infection that could turn life-threatening.
Unfortunately, while our immune system is very good at protecting us against infection - it's very bad at identifying when something is actually a true threat to the body.
When Our Immune Systems Go Haywire
The problem is that foreign microbes are far from the only thing that triggers our immune system to initiate an inflammatory response.
"Allergies" are effectively an unneeded immune response to certain organic molecules in our foods or environment. These molecules aren't actually a threat to the body - but our immune system falsely recognizes them as such. In response, our immune cells trigger an inflammatory response to attack the allergens at their entry point. The result is inflamed sinuses ("stuffy nose") and inflamed eyes and throat/esophagus.
Food allergies are particularly a problem because they're often difficult to notice (initially, anyways). Severe peanut or shellfish allergies are quite rare, but low-level allergies to foods like soy, wheat, dairy and beans are very common - and often difficult to identify because the food triggering the allergy is eaten so frequently that the low-level inflammation it triggers becomes "normal" to the person.
Toxins in our foods and environments can also trigger immune reactions, which is one reason why I place such a high priority on helping people identify sources of toxin exposure in their lives.
Stress, exposure to radiation and UV exposure are all also potential triggers of immune-mediated inflammatory response.
Acute Inflammation vs. Chronic Inflammation
When our immune cells recognize a genuine threat (invading microbe), they'll initiate an inflammatory response in the affected tissue. Macrophages and other immune cells will flood the area and eliminate the threat of infection. When the threat is eliminated, the immune response will subside and inflammation in the tissue will return to baseline. This is referred to as acute inflammation. This is the immune system working the way it's supposed to.
It is these other immune triggers - food allergens, toxins, stress - that are the real issue. These triggers are either not actual threats (allergens) or the inflammatory response is not useful in dealing with the threat (toxins).
Because these immune triggers are distributed throughout the body, the inflammatory response is not localized in the way it is with a cut or even airborne allergies. Rather, the inflammation these triggers produce is a whole-body (systemic) inflammation.
Furthermore, because we're often exposed to these immune triggers on a daily basis, the inflammatory response stays turned on indefinitely. This is what we call chronic inflammation.
What Inflammation Does To The Body
We pretty well established what inflammation is, but why is it such a such a cause for concern?
An Inflamed Body Feels Like Crap - This really should be all the reason you need to start proactively avoiding inflammation triggers in your life. Systemic inflammation creates heightened pain sensitivity in the body  and creates arthritis-like conditions in your joints . If you're accustomed to your body feeling less-than-awesome, chronic inflammation is almost certainly to blame.
Decreased Energy Levels - This is a prime symptom of systemic inflammation . The reason for it is pretty interesting: Remember that inflammation is first-and-foremost a mechanism to help our immune system deal with threat of infection. When immune cells trigger an inflammatory response, they will also down-regulate energy metabolism. Why? The prevailing theory is that this is an evolutionarily-honed mechanism designed to discourage us (or our primate ancestors) from going out and foraging during this time of vulnerability and thus being more vulnerable to predation. Pretty cool, right?
DNA De-methylation - First, let me explain what DNA methylation is and why it is important. The DNA in each of one our cells contains the information ("genes") for potentially over 1 million unique proteins. Despite this, any given cell requires only a tiny, tiny fraction of those proteins at any given time.
Fortunately, our cells have a sophisticated system to ensure that only required proteins are created (via translation). DNA methylation is an important part of this regulatory system. After a new strand of DNA is created (via replication ) in our cells, it will have methyl groups bound strategically to cytosine and adenine molecules along the strand. If DNA becomes de-methylated, unneeded proteins will be created by the cell. This not only wastes cellular energy, but potentially creates unwanted interactions that disrupt proteins that are necessary to cellular function.
DNA de-methylation is a process that happens naturally as we age. Think of it as a sort of "slow erosion" of our DNA over time that plays a big part in producing the typical symptoms of aging. In fact, the degree of DNA de-methylation is one of the most accurate measurements we have for determining a person's "biological age" (or "true" age). 
It turns out, inflammation plays a huge role in accelerating the de-methylation of DNA (and thus, aging). The same signaling molecules that instruct a cell to enter into an inflammatory state (called cytokines) also promote DNA de-methylation.  In addition to general acceleration of aging, DNA de-methylation has also been correlated with increased risk for the formation of malignant (cancerous) cells  as well as atherosclerosis (heart disease) .
Leptin Resistance - If you were wondering what I mean when I say that inflammation makes you fat, leptin resistance is the reason. Leptin is the primary hormone responsible for regulating fat storage - and telling you when you've had enough to eat (i.e. "feeling full).
When leptin is functioning properly in the body, fat storage happens at healthy levels and appetite is consistent with the calories and nutrients the body actually requires. When cells become resistant to leptin, not only does appetite increase (leading to higher caloric intake), but the calories we eat are stored as fat at a higher rate. As you might expect, leptin resistance is found nearly universally in obese people. Even in a non-obese person, how well our cells respond to leptin is a primary determining factor in how easy or difficult it is for us to maintain our desired body composition. 
Research has shown us that leptin resistance directly correlates with chronic inflammation. One of the primary proteins involved in the inflammatory response in the cell (C-reactive protein) appears to physically interact with leptin inside and outside of the cell, leading to increased leptin resistance. 
In this way, systemic inflammation is really a double edged sword. Not only will it make your body look puffy and heavier than it actually is, you'll actually be storing fat at a higher rate due to the effect on leptin resistance.
- Injury Risk - This can be a significant risk for athletes. Because inflammation brings excess fluid into a tissue, it will actually change the structure of connective tissues and muscles, making them much more prone to injury (particularly overuse injuries).