We tend to think of ourselves as individual organisms. Even though we're made up of thousands upon thousands of cells working together to send and interpret sensations, break down and rearrange nutrients, transport wastes, provide structure and mobility, and keep our selves humming along in homeostasis, we think of ourselves as single creatures.
We are not actually alone. Our bodies are home to many other organisms. Other living things go through entire life cycles on and inside us all the time. It's kind of incredible, when you stop to think about it.
I thought of this recently because a lab at UCLA is making what I think of as "people cheese." That is, cheese made using microbes collected from various people's bodies. Mouth, skin, tears, feet, and belly button microbes, to name a few.1
Those microbes didn't go directly from someone's toes into the cheese; they were cultured first. Not all bacteria culture well, so it's possible the cocktail of microbes added to the cheese does not have quite the same composition as the swab used to collect them. The principal bacteria responsible for cheese- and yogurt-making are in the genus Lactobacillus, and they're present on and in the human body.
Microbes, particularly bacteria, aren't just hanging out on our skin, though. Bacteria form biofilms on our teeth, which when allowed to harden, we call plaque. After you eat something sweet, your teeth may feel a little slimy; that's bacteria making biofilm. Brushing your teeth serves to scrub that stubborn biofilm away.2
Bacteria can also be beneficial. There's a craze for "probiotics," but your body probably already contains many beneficial bacteria in your gut. Other folks in my lab study Bacteroides thetaiotaomicron, one of a family of bacteria responsible for providing up to 10% of your energy. They break down starches into simpler sugars that your body can digest. Microbes like "B. theta" are why you can feel a little "off" after taking antibacterials.3 You may wipe out the good bacteria along with the bad.
Many diseases are caused by unhelpful bacteria. I study one of them, Vibrio cholerae, which causes cholera. Like B. theta, V. cholerae are happy to live in your gut. Unlike B. theta, they are not welcome to stay.
There are other microbes besides bacteria that live in you, too. Yeast (e.g. Candida albicans), for one. Normally yeast and bacteria can balance each other out, but when that balance is disrupted, the results can be unpleasant. Protozoa (e.g. Plasmodium spp., Giardia lamblia) can also make themselves at home in your body.
So you're never really alone. You've got lots of company: about ten times as many bacterial cells as your own human cells, along with many other organisms that call you home.4
1: It puts me in mind of Harry Potter and the Chamber of Secrets, when the trio of protagonists are considering Polyjuice Potion. Essence of Goyle, anyone?
2: Yes, I've had some pretty great conversations with my dentist about petrified biofilms.
3: Antibacterials are frequently referred to as "antibiotics," but that name implies they are effective against viruses and bacteria. "Antibacterial," while a less common term, is also less prone to misunderstanding. For more on "antibiotic" vs. "antibacterial," see Colin Purrington's words on the subject.
4: Even though they outnumber you by about an order of magnitude, they account for only 1–3% of your bodyweight (according to the Wikipedia page, which cites an NIH project). Yeah, bacteria are tiny. Red blood cells are on the smaller side of human cells at around 9 µm in diameter, but a V. cholerae cell is 2–4 µm long and less than 1 µm wide. There's a neat site for showing the scale of many things, including bacteria and human cells, here (requires Flash).