Invasion of the body snatchers: billions of microscopic critters have taken over your skin, hair, and gut. Here are five that might make you scream ... If only you could see them.

DID YOU KNOW?

* The skin of the average-size human--which covers an area of 1.7 square meters (2 square yards)--is home to as many bacteria as there are people in the United States. The U.S. population is estimated at 294 million people.

* The growing popularity of eating sushi has caused a rise in human infection from the broad fish tapeworm (Diphyllobothrium latum). This parasite, which grows to 18 m (60 feet)-long, infects tiny freshwater crustacean and fish, and certain migratory fishlike salmon. Humans can catch the worm when they eat infected raw fish, leading to severe digestive problems.

CRITICAL THINKING:

* Ask your students if they think they could survive if their bodies were free of bacteria and microbes.

CROSS-CURRICULAR CDNNECTIONS:

HISTORY Read aloud passages from "Body Beasts," by Richard Conniff, National Geographic, December, 1998. (www.nationalgeographic.com/ngm/9812/fngm/) Then, have students research and report on how parasite infestation was once a normal part of life.

RESOURCES

* Grolier search term: bacteria

* Find information about head lice, see: www.headlice.org.

* This film looks at the microorganisms that live inside a house: ABC World of Discovery: The Secret Life of 118 Green Street, ABC Home Video, 1993.

The feast begins on your face as soon as the lights go out. Tiny, eight-legged relatives of the spider crawl out of your skin. They scamper from hair to hair searching for a mate. Then, in the morning, they burrow face-first back into your skin. Sound like a scene from a horror movie? It's real.

About one out of every four of your classmates has follicle mites, or Demodex folliculorum (DEH-muh-deks foh-LIK-yoo-LO-nun), on his or her eyelashes and skin. "They're normal inhabitants of the skin," says Serena Mraz, a dermatologist at Solano Dermatology Associates in Vallejo, California. You can contract the parasites (organisms or nonliving particles that depend on another organism to exist) by snuggling--or even just sharing a towel--with someone who is host to the miniscule monsters. Once they're on you, these mites hang out at the base of hair follicles, or pores where hairs grow. Here they eat the fatty oils that spurt from sebaceous (sub-AY-shus) glands beneath your skin's surface. "And as gruesome as these guys may appear under the microscope, there's no way to eradicate them," Mraz says.

Worse, they are not alone. Your skin, hair, and gut continually crawl with microscopic bacteria (single-cell organisms), fungi (FUN-guy, single cell or multicellular organisms such as mold and yeast), and viruses (nonliving particles that invade and reproduce in a living cell). "Bacterial cells alone outnumber our body's cells by 10 to 1," says Abigail Salyers, a microbiologist at the University of Illinois, Urbana-Champaign. They feed on dead skin, body fluids, and each other. "We're a free lunch," she says.

Thankfully, only a few of these organisms are harmful. Read on to meet some of these creatures.

ARMPIT LOVER: Staphylococcus aureus (staf-ee-lo-KAH-cus OR-ee-us)

FAVE HANGOUT: Moist armpits

SCREAM-WORTHY TRAIT: Eats sweat and produces a stinky chemical known as body odor.

HOW TO GET RID OF IT: You can't. But deodorant stops the stench.

THE NITTY GRITTY: Whether they're clean-shaven or hairy, the warm, moist areas under your arms feed millions of these sweat-munching bacteria. Fresh sweat doesn't smell like much. But Staphylococcus aureus, or staph, breaks down the mixture of salts and oils in sweat. What's left is a foul-smelling chemical. "They're not [making you stink] on purpose. They're trying to create nutrients and energy," says Salyers. "The odor is just a side effect."

The good news? Antiperspirant plugs sweat glands to dry up your pit juice and prevent excessive amounts of bacteria from growing. And don't underestimate the power of a shower, which helps keep the bacteria in check. But you wouldn't want completely bacteria-free pits: The bacteria form a protective barrier that helps keep fungi from taking over your skin.

FOOT FUNGUS: Dermatophytes (der-MAT-o-fites)

FAVE HANGOUT: Between toes

SCREAM-WORTHY TRAIT: Eats dead skin and causes itchy, red patches known as athlete's foot.

HOW TO GET RID OF IT: Beat an infection with antifungal cream and clean socks.

THE NITTY GRITTY: Microscopic mold is everywhere--from your gym towel to your breakfast cereal. In fact, dermatophytes, three species of flesh-eating fungi that only live on people, are probably gnawing at your toes right now!

"[Dermatophytes] feed on a substance in the skin called keratin," says Dr. Luis Ostrosky-Zeichner, a fungi expert at the University of Texas Medical School in Houston. Keratin is the protein (chemical made of chains of compounds containing carbon, hydrogen, oxygen, and nitrogen) that makes skin stretchy. Lucky for you, dermatophytes are only interested in dead skin cells. And since every year the average person sheds about half a kilogram (1.1 pounds) of dead skin, you should thank the skin snackers. Without them, the world would be piled high with your fleshy flakes.

In rare cases, dermatophytes--which prefer the moist pockets between your toes--can grow unchecked and leave your feet fuzzy with mold. "It's not like growing mushrooms (edible type of fungus) on your feet, but they start to make strange shapes," says Ostrosky-Zeichner. The most common dermatophyte infectious cause a person's feet to shed more and more skin. The fungi thrive in sweaty socks and damp shoes--leading to constant peeling and itchy sores.

BUMPY VIRUS: Papovavirus (pa-PO-vuh-virus)

FAVE HANGOUT: Hands

SCREAM-WORTHY TRAIT: Can lurk in your body for months before you even know you're infected.

HOW TO GET RID OF IT: Scientists don't know.

THE NITTY GRITTY: Like a horror-movie villain, papovaviruses lurk unseen. But they're always there. Ninety-five percent of all people are infected with this virus, which can cause skin to grow into tumorlike warts. Unlike many body snatchers, viruses have no proven benefits to people. "Bacteria and fungi are free-living. But viruses are completely parasitic. They have to invade a living cell [to reproduce and grow]," says Salyers.

Papovaviruses are often passed by a handshake. The virus inserts itself into a skin cell's DNA (chemical carrying hereditary information) and uses the cell to replicate (reproduce). When too many cells are invaded, the infected patch forms bumps.

Most people who have the virus never get warts, and scientists don't know why. But you're five times more likely to get them if you're a 9- to 16-year-old girl. Luckily, doctors can remove warts from your skin. Left alone, the bumps may take years to disappear.

HAIRY INSECT: Pediculus capitus (PED-ik-YOU-lus KAP-ih-tus), or head lice

FAVE HANGOUT: Hair

SCREAM-WORTHY TRAIT: Each bloodsucking female lays up to 100 eggs.

HOW TO GET RID OF IT: Kill lice with pesticide shampoo.

THE NITTY GRITTY: The key to avoiding these wingless insects: Don't share hats or hair brushes. That's because these villains nest in hair, where they suck blood from their victim's scalp. Worse? Swarms of lice drop feces and saliva on your skin. That leaves you more sensitive to their bites and makes your head itch like crazy.

Unlike many of the body's other invaders, head lice aren't permanent residents. For the 6 to 12 million people--mostly kids--who get them each year, special pesticide-containing shampoos can poison these parasites.

Lice aren't microscopic either. Look closely and you'll see their brown and gray bodies crawling around a bug-ridden scalp. If left untreated, an infested person's scalp can be as busy as Times Square on New Year's Eve.

NUTS & BOLTS

A close relationship between two organisms of different species is called symbiosis. There are three types of symbiotic relationships:

* Mutualism: Both organisms benefit from the partnership.

* Parasitism: One organism benefits; the other is harmed.

* Commensalism: Only one organism benefits; the other is unharmed.

Can you identify one example of each type in the article?

Name: --

INVASION OF THE BODY SNATCHERS

DIRECTIONS: On a separate piece of paper, defend or dispute the following statements. (Hint: Defend means to explain why a statement is correct. Dispute means to explain why a statement is incorrect.)

1. Sweat causes body odor.

2. It is important to use an antibacterial soap to remove all traces of bacteria from your skin.

3. Head lice are not permanent residents on an infected scalp.

4. Dermatophytes are beneficial, but can become a nuisance,

ANSWERS

1. Sweat does not cause body odor. The Staphylococcus bacteria on your skin break down the salt and oil in sweat for nutrients. The side effect is a foul-smelling chemical.

2. You do not want all bacteria removed from your skin, The bacteria form a protective barrier that helps keep fungi from taking over your skin.

3. Head lice are not permanent residents. You can get rid of head lice by using special shampoos that can poison these parasites with pesticides.

4. Dermatophytes are beneficial, because they eat dead skin. If they grow unchecked, especially in the moist areas between your toes, they will leave your feet fuzzy with mold. The infection-which becomes peeling, itchy sores-causes a person's feet to shed more and more skin.

EYE SPY: A follicle mite's mouthparts can pierce right through your skin.

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NO FEAR: Follicle mites are harmless.

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HOME BASE: Your eyes and pores house too many follicle mites to count.

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ARMPIT LOVER: Every square centimeter of your armpit houses about 2 millon microbes--mostly bacteria.

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SORE FOOT: Athletes--with sweaty socks and damp shoes--are often victims of dermatophytes. Hence the term "athlete's foot."

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BUMPY: Of all the people who have papovaviruses, only 7 to 10 percent will have warts at any given time.

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HEAD-LICE NURSERY: A louse's nits, or eggs, (oval shape) cling to individual hairs like dandruff.

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