Given how much money we spend on wrinkle remedies, bronzing creams and mud facials, you'd think our skin is mere decoration. But disturbing as this may seem to you readers with disinfecting wipes in your pockets, the normal human epidermis doubles as a complex ecosystem every bit as vibrant as the Amazon rainforest or the African Serengeti.
Our external surface area, which ranges from an average of 1.6 square meters in adult women to 1.9 square meters in men, is populated by a vast number of other life forms [source: Medterms.com]. Just how vast? Your hide is home to approximately 1 trillion bacteria, which is about 150 times the entire human population of our planet [source: Todar]. That population includes close to 500 different bacterial species, including 44 that live in the neighborhood that covers your forearm and another 19 who own real estate behind your ears [source: Science Daily, Science Daily]. That's not necessarily a bad thing, either. Some of the bacteria on your skin perform beneficial functions, like preventing excessive inflammation after injuries to the skin [source: Warner].
Additionally, you may have scores of other tiny tenants, from the mites that live in your eyebrows, to fungi and small insects. Some are harmless, while others cause unpleasant but minor skin ailments, like acne or jock itch. But more than a few can cause potentially serious illnesses, such as antibiotic-resistant infections or Lyme disease. So, we need to be careful what creatures we host.
Here's a look at 10 life forms crawling around on our skin.
We like to think we're pretty special, but to bacteria, we're just Petri dishes. The microbes on and inside our bodies actually outnumber our own cells by a factor of 10 to one [source: Harris]. Bacteria staked their claim on you even before you were born, colonizing your skin as you passed through your mother's birth canal [source: Todar]. And you can never totally get rid of the resilient little homesteaders, who find inviting havens in the moist, warm crevices of your anatomy, no matter how often you wash. Even an area as exposed as the inner fold of your elbow, for example, is host to about 1 million bacterial cells per square centimeter of skin. (If that grosses you out, then you don't even want to hear about your groin and armpits.)
The lifetime of a single microbe on your skin may be brief -- millions of them die off each day -- but the colonies in which they reside survive for a long time, cloning themselves to create replacements. But don't worry too much. Scientists say that most of your bacterial inhabitants are harmless, and some are even good for you. The bacteria on your elbow, for example, help process the raw fats that your skin produces, continually giving you a natural moisturizer treatment [source: Wade]. But other bacteria are less friendly. Methicillin-resistant Staphylococcus aureus (MRSA) can cause sores or boils on the skin, and if it manages to infect a surgical wound and get into your bloodstream, it can trigger a potentially life-threatening infection. People who are crammed together in tight spaces and have a lot of skin-to-skin contact or touch the same objects -- athletes, prison inmates, military recruits, hospital patients and even people who work out at fitness centers -- are particularly vulnerable to catching MRSA infections from each other. One of the scariest things about MRSA is that it continually adapts to become resistant to whatever antibiotics doctors use against it [source: WebMD].
Bedbugs, which belong to the family Cimicidae, are less than a quarter of an inch long, with flattened, wingless rust-colored bodies. But don't let their miniscule size and bland appearance fool you. These exceedingly unpleasant creatures, who hide out in mattress seams, box springs, cracks between bed frames and couch cushions, are basically tiny vampires, waiting for an opportunity to jump onto your skin, bite you and drink your blood.
It's nothing personal, mind you. To them, you're a source of nourishment -- nourishment they need to molt into their next life cycle. Bedbugs don't stay long; typically they feed for less than 10 minutes. But during that time they gorge on six times their original body weight in blood. While they aren't known to transmit any disease-causing pathogens, bedbug bites do cause red raised areas called papules, which are painfully itchy and may lead people to think you've been staying at a sleazy motel [source: Bohart Museum of Entomology].
Unfortunately, the truth is that bedbugs infect five-star lodging establishments, too. While powerful pesticides such as DDT had bedbugs on the run decades ago, they now seem to be on the rise globally. Some blame the increase in international travel -- bedbugs love to hitchhike in your dirty laundry as you roam -- and the relative ineffectiveness of newer, more environmentally friendly anti-pest products [source: Sharkey].
Of all the parasites that view your skin as a smorgasbord, the maggots of the human botfly (Dermatobia hominis), which prey on people in South and Central America, may be the most repulsive offspring this side of the extraterrestrial monster's eggs in the sci-fi thriller "Alien." But at just a few centimeters in length, at least they're not big enough to attach themselves to someone's face and eat the entire thing.
A relative of the parasite that afflicts horses and dogs, the human botfly has an odd method of reaching you; females capture mosquitoes and other blood-feeding insects and glue their eggs to them, so that when the released insects feed on your skin, they implant botfly eggs there as well. The result is an unsightly, painfully hard, pus-secreting bump in your skin, underneath which a young botfly maggot is developing. In some cases, victims actually can feel the botfly moving under their skin.
Unless you've always dreamed of running a botfly farm on your hide for the three months it'll take the maggot to develop into a fly, your best bet is to see a doctor. The removal process is nearly as gross as the botfly maggot itself. Since a botfly maggot breathes through spiracles that are flush with the skin, a doctor will cover the skin with beeswax or another thick substance to smother it. When the maggot comes up out of the lesion to gasp for breath, the doctor grabs it with forceps and yanks it out. There are a number of YouTube videos demonstrating the process, but be sure not to watch them too soon after eating [source: Larrick and Connelly].
7: Follicle Mites
If the thought of little bugs infesting your eyelashes horrifies you, don't read the rest of this page. There are two species of Demodicidae, or follicle mites, which infest humans. Demodex folliculorum, the larger of the two, is an oblong, eight-legged bug that ranges from 0.2 to 0.4 millimeters in length, which is just the right size to comfortably inhabit the follicles of your hair and eyelashes. Up to 25 of the bugs can squeeze into a single follicle. Its smaller cousin Demodex brevis, which measures between 0.15 and 0.2 millimeters, prefers less cramped quarters within the skin's sebaceous glands.
Both of them share an appetite for the insect world's equivalent of greasy-spoon fare -- sebum, the waxy, fatty goo secreted by your skin's sebaceous glands. It's not that difficult to get rid of Demodex infestations. Repeated daily washing with a baby shampoo, diluted with antibiotic cream, often does the trick. But the little buggers often escape detection because of their miniscule size.
Humans transfer the mites to one another -- the initial infestation often occurs when mothers nurse their infants. And the incidences increase with age. By one estimate, upwards of 90 percent of elderly people are host to them. The only time hosts notice the mites' presence is when they cause skin eruptions such as acne or Rosacea [source: Service].
Fleas, who belong to the order Siphonaptera, have a nasty reputation, in part because they hang out on loathsome creatures like rats (and humans), but mostly because they transmit microbial diseases. The most frightening example was the dreaded bubonic plague, which killed millions of Europeans during the Middle Ages.
Fortunately, those of us who are lucky enough to live in modern industrialized countries have antibiotics to protect ourselves against plague epidemics. But the flea-borne ailment still infects between 1,000 and 3,000 people in other parts of the world annually [source: CDC]. There actually is a flea specific to humans, Pulex irritans, but it's relatively rare. You're most likely to be bitten by the cat flea, Ctenocephalides felix, or by therat flea, Xenopsylla cheopis.
All of these creatures are flat, wingless insects that travel like hobos on the railroads, jumping from one human host to another. They have a springlike mechanism in their bodies, which makes them the Michael Jordans of the insect world, able to cover distances of up to 100 times their body length. Female fleas lay eggs in areas where animals sleep, and the larvae subsist on organic debris until they reach adulthood and develop a taste for blood. Washing bedding and carpets, killing vermin and spreading insecticide around are the usual flea remedies, but frankly, they don't always solve the problem. One reason it's so difficult to get rid of fleas permanently is that they're hardier than the typical "Survivor" contestant. They actually can go as long as two years between blood meals [source: Bohart Museum of Entomology].
The 600 to 700 species of leeches, of the subclass Hirudinea, are hermaphroditic segmented worms with suction cups at each end. They're found mostly in freshwater habitats around the world -- that is, when the little carnivores aren't busy latching onto someone's skin with their teeth and sucking the person's blood [source: Myers].
They're also the only creature on this particular list of skin crawlers that people attach to their skin on purpose. Dating back as far as 7th-century-B.C. India, healers put leeches on ill people and let the leeches feed, in the erroneous belief that getting rid of an overabundance of blood would put the body back in balance [source: Michalsen]. With the advent of modern medicine, using leeches fell out of favor, freeing us to concentrate on their loathsome ambiance. While leeches can be as small as 7 millimeters in length, a specimen of a recently discovered species, the large-toothed South American Tyrannobdella rex, measured 44.5 millimeters [source: Australian Museum]. Science Daily reports that the creature was plucked from the nose of a young girl who'd been bathing in an unnamed Peruvian river [source: Science Daily].
These flat-bodied, wingless insects are 2 to 3 millimeters in length and feast on human blood. Though body lice have been a carrier of the organism that causes typhus, a deadly fever, you're unlikely to catch it unless you live in a crowded city in an underdeveloped country; instead, when they bite your scalp or groin, you'll just experience a lot of redness and an uncomfortably persistent itchiness [source: Medscape].
You may also experience a case of embarrassment, since lice infestation is strongly associated in American culture with poverty and/or lack of cleanliness. Like most stereotypes, though, that's not really a fair assumption, since as many as 12 million Americans become infested each year, and kids at upscale daycare centers get lice just like everyone else [source: eMedicine Health]. Truth is, the little buggers -- the lice, that is -- have legs that are ingeniously adapted for traipsing through any sort of human hair or on clothing. There are three different species that prey on humans: the human head louse (Pediculus humanus capitis), body louse (Pediculus humanus humanus), and crab or pubic louse (Phthirus pubis). All three of them are bugs on a mission because if they don't feed every 24 hours, they'll starve to death. That's bad news for you, if you happen to come in contact with them. Getting rid of lice involves multiple treatments with an insecticidal shampoo and rooting through the hair with a long-toothed comb.
The skin disorder scabies is caused by Sarcoptes scabiei, a round mite with a spiny, striated body and eight short, conical legs. Given that it's just 0.3 to 0.45 millimeters long, this mite inflicts a disproportionate amount of pain [source: Service]. When the female Sarcoptes scabiei lands on you via skin-to-skin contact with an infected person or that person's clothing or bedding, the mite tunnels horizontally into your epidermis and deposits its eggs there. After three days, the eggs hatch, and the developing larvae move to your hair follicles. It takes the precocious parasites just two weeks to become mature mites. The mites then may roam anywhere on your skin, but they're particularly fond of skin folds, such as the little webs between your fingers.
Coincidentally, at about the two-week mark, you'll feel a painful itching sensation and see dark or gray stripes on your skin over the locations of the tunnels. Infestation by Sarcoptes scabiei can really mess up your skin with swelling and flaking, but some of the worst damage is self-inflicted. You feel compelled to scratch your skin so often and so hard that you cause superficial ulcerations, which in turn can become breeding grounds for secondary bacterial infections. Fortunately, you can rid yourself of the loathsome mites through frequent bathing followed by application of an insecticidal lotion, and by disinfecting your towels, clothing and bedding [source: Kruger and Botha].
You're probably familiar with the concept of hoards of zombies roaming the countryside, ravenous for a meal of human flesh. Well, think of the maggots from the two species of screwworm flies -- the western hemisphere's Cochliomyia hominivorax and the eastern hemisphere's Chrysomya bezziana -- as miniature flesh-eating zombies.
While screwworms are mostly thought of as a threat to livestock, these larval invaders will feast ravenously on the living tissue of any mammal, including you -- provided that you're unlucky enough to suffer an open wound and don't promptly receive proper medical attention, including sterile bandaging.
The adult screwworm fly is about twice the size of a regular housefly, with a greenish-blue body and big reddish orange eyes. After mating, the female looks for a gash in the skin of an animal or human in which it can lay its eggs -- up to 2,800 of them during the female's month-long lifespan. With your torn flesh providing a nice warm, moist nest, a screwworm fly egg can hatch in as little as 12 hours, and within a week of eating you alive, it grows into a larva that's more than a half-inch long. At that point, it's "hasta la vista" for the larvae, which drops from your body and tunnels into the soil to form a protective case for the pupa from which an adult screwworm fly eventually will emerge. By then, though, you're likely to be a corpse, since screwworms can kill a full-sized steer in that length of time [source: Indiana Animal Disease Diagnostic Laboratory].
1: Blacklegged or Deer Tick
Ixodes scapularis, which is found in the eastern United States, and its cousin Ixodes pacificus, are nasty little brown and black arachnids who like to hitch a ride on your body while you're strolling in blithe bliss through the great outdoors. These ticks are feared primarily because they spread Borrelia burgdorferi, a bacterium that causes Lyme disease, a dreaded inflection that weakens the victims' muscles and can lead to abnormal movements, numbness, tingling and speech problems, in addition to making him or her feel like crud [source: PubMed].
Though the ticks are commonly known as deer ticks, you don't really need to be in close proximity to Bambi to find them. The ticks have a two-year life cycle, during which they go through three stages: larva, nymph and adult tick. After adults lay eggs on the hide of a host animal, the eggs hatch and release larvae, which drop into the grass or bushes and wait to catch a ride on some small animal, such as mice or chipmunks. If a larva feeds on the blood of an animal already infected with Lyme disease, it'll become a carrier, and then bring the bacteria along when it grows to the next stage and becomes a nymph. The nymphs, which are about the size of poppy seeds on bagels, are the ones that you have to worry about because they attach themselves onto slightly larger mammals -- squirrels, raccoons, dogs, cats, and unfortunately, people -- and infect them with Lyme disease. The biggest risk comes in the summer months of June and July, when the nymphs are active. You can also get Lyme disease from an adult tick, but they're a little less dangerous because they're bigger and easier to spot and remove.
If you're unlucky enough to be bitten and become infected, most of the time you'll develop a spreading "bulls-eye" rash from the region of the tick bite within two to four weeks, and experience flulike symptoms, including headache, and pain and swelling in the joints [source: University of Rhode Island].
Lots More Information
- Mysterious Bacteria Quiz
- Skin parasites
- Lyme Disease
- 5 Parasites That Breed On and In Your Skin
- Surviving Parasites Videos
- "The African Queen." Turner Classic Movies. (Oct. 10, 2011) http://www.tcm.com/this-month/article/153609%7C0/The-African-Queen.html
- "Bedbugs." Bohart Museum of Entomology. (Oct. 10, 2011) http://delusion.ucdavis.edu/bedbugs.html
- "Botfly Infection." Animal Planet. (Oct. 10, 2011) http://animal.discovery.com/invertebrates/monsters-inside-me/human-botfly-dermatobia-hominis/
- "Deer Tick Ecology." American Lyme Disease Foundation. Jan. 5, 2010. (Oct. 11, 2011) http://www.aldf.com/deerTickEcology.shtml
- "Deer Ticks." University of Rhode Island Landscape Horticulture Program. 1999. (Oct. 11, 2011) http://www.uri.edu/ce/factsheets/sheets/deerticks.html
- "Definition of Body Surface Area." Medterms.com. April 3, 2008. (Oct. 7, 2011) http://www.medterms.com/script/main/art.asp?articlekey=39851
- "Epidemic Typhus." Medscape. 2000. (Oct. 11, 2011) http://www.medscape.com/viewarticle/410104_2
- "Fleas." Bohart Museum of Entomology. (Oct. 10, 2011) http://delusion.ucdavis.edu/fleas.html
- Harbin, L.J. Etal. "A sebaceous cyst with a difference: Dermatobia hominis." Journal of Clinical Pathology. October 2002. (Oct. 10, 2011) http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1769786/
- Harris, Kenneth. "Worlds of Bacteria, Alive On Your Skin." NPR. May 28, 2009. (Oct. 7, 2011) http://www.npr.org/templates/story/story.php?storyId=104662183
- "Humans Have Ten Times More Bacteria Than Human Cells: How Do Microbial Communities Affect Human Health?" Science Daily. June 5, 2008. (Oct. 9, 2011) http://www.sciencedaily.com/releases/2008/06/080603085914.htm
- Kruger, T.F. and Botha, M.H. "Clinical Gynaecology, Third Edition." Juta and Co. 2007. (Oct. 11, 2011) http://books.google.com/books?id=mEsPakNJWZYC&pg=PA142&dq=Sarcoptes+scabiei&hl=en&ei=aVeUTrPyHenm0QHb7LTHBw&sa=X&oi=book_result&ct=result&resnum=2&ved=0CEkQ6AEwAQ#v=onepage&q=Sarcoptes%20scabiei&f=false
- Larrick, Stephanie K. and Connelly, C. Roxanne. "Common name: human bot fly, torsalo (Central America), moyocuil (México), berne (Brasil), mucha (Colombia), mirunta (Perú), and ura (Argentina, Paraguay, and Uruguay)." University of Florida Department of Entomology and Nematology. July 2008. (Oct. 10, 2011) http://entnemdept.ufl.edu/creatures/misc/flies/human_bot_fly.htm
- "Leeches." Australian Museum. June 4, 2010. (Oct. 10, 2011) http://australianmuseum.net.au/Leeches
- "Lyme Disease." PubMed Health. Aug. 26, 2011. (Oct. 11, 2011) http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0002296/
- Michalsen, Andreas; Roth, Manfred; and Aurich, Michael. "Medicinal Leech Therapy." Appl Wemding. 2007. (Oct. 10, 2011) http://books.google.com/books?id=MwlmGPHg2CsC&pg=PT138&dq=medicinal+use+of+leeches&hl=en&ei=g5uTTrDpOIXr0gGBhKjRBw&sa=X&oi=book_result&ct=result&resnum=3&ved=0CEYQ6AEwAg#v=onepage&q=medicinal%20use%20of%20leeches&f=false
- "The New T. Rex: A Leech With an Affinity for Noses." Science Daily. April 15, 2010. (Oct. 10, 2011) http://www.sciencedaily.com/releases/2010/04/100414184205.htm
- "Plague." CDC. June 25, 2009. (Oct. 10, 2011) http://www.cdc.gov/ncidod/dvbid/plague/
- "Scabies." PubMed Health. Oct. 4, 2010. (Oct. 11, 2011) http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001833/
- "Screwworm." Indiana Animal Disease Diagnostic Laboratory newsletter. Fall 2000. (Oct. 11, 2011) http://www.addl.purdue.edu/newsletters/2000/fall/screwworm.shtml
- Service, Mike. "Medical entomology for students." Cambridge University Press. 2008. (Oct. 10, 2011) http://books.google.com/books?id=wRrof4RLDuwC&pg=PA255&dq=Demodex+folliculorum&hl=en&ei=Zm2TTt-eKaO80AHV-YEZ&sa=X&oi=book_result&ct=result&resnum=7&ved=0CFIQ6AEwBg#v=onepage&q=Demodex%20folliculorum&f=false
- Sharkey, Joe. "A Pest From Yesteryear, Bed Bugs Travel Nowadays." The New York Times. July 13, 2009. http://www.nytimes.com/2009/07/14/business/14road.html
- Todar, Kenneth, Ph.D. "The Normal Bacterial Flora of Humans." Todar's Online Textbook of Bacteriology. 2008. (Oct. 10, 2011) http://textbookofbacteriology.net/normalflora_3.html
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- "Unexpected Bacterial Diversity On Human Skin; New Approaches For Treating, Preventing Skin Diseases." Science Daily. May 29, 2009. (Oct. 9, 2011) http://www.sciencedaily.com/releases/2009/05/090528142821.htm
- Wade, Nicholas. "Bacteria Thrive in Inner Elbow; No Harm Done." The New York Times. May 23, 2008. (Oct. 7, 2011) http://www.nytimes.com/2008/05/23/science/23gene.html
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