It's really impossible to discuss the human biology without first touching upon the scale of human beings. They are massive creatures. Epic by many standards. Size, of course, is relative. Compared to a planet, they're minuscule. But compared to most common life forms- well, let's put them in context.
Earth does things large, when it comes to life. Perhaps it's a side effect of the weak gravity of this tiny terrestrial rock, but things grow to enormous proportions on Earth. Titanic mammals that can only exist suspended in the world's water oceans. Forests that share common root structures and are like one gigantic tree. Compared to these, humans are modestly sized, but these organisms are outliers. Compared to the most common forms of life- well, humans and whales are on the same order of magnitude.
All life on Earth starts with proteins. Complex carbon molecules that can form long chains (hey, I told you they like everything large) bind up into interesting chemistry. The simplest forms of life are parasitic bundles of protein called viruses. They can get fairly large for protein crystals, but for terrestrial life, are about as small as it gets. This economy of size is offset by some major failings- they need other organisms to make their lifecycle work.
The bulk of all life on Earth is organized into cells. While there's a lot of variation in size, the same basic model is used across all cells. At the center, the complex proteins that carry inherited information. A water based jelly provides suspension for all the various organelles in the cell; once again, mostly proteins. The whole thing is wrapped up in a lipid bubble that provides a membrane to keep the cell's parts in and the world out. This membrane-barrier life strategy is unique to Earth, and itself provides curious insights into why life behaves the way it does. But that's for a later article.
This lipid-wrapped bundle of protein and water is about 10µm across, and they're everywhere. Earth teems with these cellular organisms. The air, the water are filled with them. Every surface is coated with them. And most of these organisms are just one cell.
Humans are made up of these cells, but they are made up of about 1014- 100 trillion cells. For another comparison, a single cell is 10µm across. Humans, on their longest axis, are about 2 meters across- 106 times larger. In contrast, the Earth is about 6 megameters- about 106 times larger than a human.
Think about that. Single celled organisms are to humans as humans are to their planet.
Imagine such a creature, perhaps living in a puddle of water. A human walks by and carelessly steps in that puddle. The resulting disruption is geological in scale. While the single celled organism has little awareness of what has transpired, its environment has been devastatingly altered. The water displacement causes turbulence that reorganizes all of the life in the pool. Sediments are stirred up, clouding the water (a hindrance to the many photosynthetic lifeforms that could be there). Cells from the human become part of that sediment (they constantly shed dead cells). Living cells, carried their on the human's skin may join the pool. Maybe they are prey, perhaps they are predator. Either way, an incredibly alien event has transpired.
The analogy of size, Cell:Human::Human:Planet is an excellent analogy. Humans don't interact with the single-celled world merely by crashing through puddles. They are so large that they constitute an entire ecosystem. Their body is more than just a collection of human cells; it's a breeding ground for other organisms, ranging in size from single celled organisms to much larger and vaster insects (themselves large enough to carry hosts of smaller organisms).
Like any other ecosystem, humans and their occupants live in careful balance. Some are mere parasites, ranging from a host of bacteria that excrete chemicals harmful to humans, to blood sucking insects or worms that feed on human digestive efforts.
Most, however, are welcome occupants that provide valuable services to their host planet. Nearly every orifice plays host to colonies of bacteria. In females, for example, their genitals depend on a healthy colony of bacteria to function successfully. A variety of maladies can be caused by an imbalance on those colonies. In both sexes, one of the more critical colonies is in the large intestine. These play a vital role in human digestion; they help extract vital nutrients from the food humans eat and make them accessible to the human body.
There are others that are generally welcome or ignored, until they attack their environment in some way. The skin, for example, is coated in bacteria that do little to contribute to their environment, but also do little to harm it. Occasionally though, these colonies grow too aggressively, and interfere with the human skin's own maintenance processes- the result is ugly and painful blemishes that are attacked by the human's immune system.
This bears repeating: humans are so large that they are coated inside and out with all manner of bacteria and other single celled creatures. Arachnids and insects are the giants in this universe; they live off of the dead skin and dust that collects on the humans. The human body is not a single organism, nor even a single ecosystem, but a collection of very different environments that are the stage for thousands of species to live out their lives. Some live in harmony with their environment, some in ignorance of it, and a few actively try and profit from it.
There's one more denizen of the human body that I want to mention. You see, most human behavior appears to arise within the human brain- a collection of highly specialized cells that work as a massively networked computer system (these cells are among the larger cells out there- they grow long arms to build this network). It's very tempting to think that much of their behavior and personality arises there. This is foolish because of the many, many systems in their body that are more powerful motivators, but that's for another essay. You see, there are some unicellular creatures that actually know how to "hack" the brain's network. They can actually control, to a limited extent, the personality and behaviors of a human being. Some drive them mad. Some are more benign, and merely try and make the humans good prey for their preferred host. In either case, there is not a single system in the human body that is not somehow dependent on, host to, or manipulated by some smaller, more humble organism.