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The sun is setting over a farm near a tropical rain forest. The cows and horses begin to doze. Hundreds of tiny bats fly out of their caves. The bats hear the sound of the animals quietly breathing. They focus in on their prey. These are vampire bats—and they are out for blood.

People in the steamy tropics of Central America and South America are familiar with vampires. It's not the Dracula type of vampire, but the bat type. Vampire bats are small, no bigger than a mouse. Don't let their small size fool you, though. A vampire bat's teeth are razor sharp.

The bat uses its teeth to clip away a bit of an animal's hair or fur. That leaves a small patch of skin. The bat then makes a tiny cut in the skin, about the size of a pinprick. The wound bleeds one small drop of blood at a time. With its tongue, the bat laps up about two tablespoons of blood during a feeding. Through all of this, its victim usually sleeps and never feels a thing!

Eating blood may seem like an odd way to survive. Yet vampire bats aren't the only animals that are into hematophagy. That means feeding on blood. While this eating habit may seem gross or downright dangerous, blood is rich in nutrition and easy to find. That's why hematophagy is a way of life for many creatures. Blood feeders live just about everywhere—even near you!

While vampire bats live only in the tropics, leeches live all over the world. Freshwater leeches are worm-like animals with a big mouth. Well, two mouths, actually—one at each end of its body. Each mouth is a powerful sucker that attaches the leech to its prey. Fish and reptiles are their usual prey, but a human works fine, too.

The leech uses its sharp teeth or needle-like proboscis to pierce skin before it sucks the blood. At one feeding, a leech draws out several times its own weight in blood. Once it is full, the leech drops off.

A leech bloated with blood is not a pretty sight. In fact, it's pretty gross. But they're not all bad news. Just ask your doctor. Doctors sometimes put some kinds of live leeches on a patient's skin to stop swelling.

Also, leeches make a chemical that stops blood from clotting. Scientists are finding ways to use this chemical to treat heart disease. So you see, a leech may be a lot of mouth, but it isn't all bad.

Some bloodsuckers, such as mites and bedbugs, can be found in people's homes—ugh. Mites can hide anywhere in the house because they are so small. Most of them are too tiny to see with your eyes.

Though not all mites are bloodsuckers, most are. The good news is that only a few mites bite humans. Most prefer small animals or insects. Still, blood-sucking mites sometimes enter a house on an insect or pet. Their bite isn't dangerous, but it can cause an itchy rash.

Bedbugs prefer warm, cozy places near their prey. Spaces near mattresses make an especially good home for bedbugs. When they come out at night to feed, a sleeping body is right there.

Bedbugs are about as big as apple seeds, but expand as they drink several times their weight in blood. Their bites leave itchy bumps on the skin, but they do not carry disease. Even so, no one wants to share a bed with a bug!

Hikers cannot say enough bad things about ticks! Ticks are arthropods, like spiders and scorpions. Ticks thrive in wooded areas, clinging to the bottom of plant leaves or blades of tall grass. Ticks cannot jump or fly. They can only crawl—and wait.

A tick waits for days, even years, for a host. When an animal or person brushes past, the tick grabs on and goes to work. It burrows its small head under the skin to feed. A tick takes several days to drink its fill. The host may not even know the bug is there.

A tick bite can be dangerous. Ticks may carry diseases that they pass on to their hosts. So take care! When you go hiking, try not to brush against tall grass or bushes. Wear white socks over your pants, because ticks are easy to spot on white. Check your clothes for these unwanted hitchhikers after a hike. You don't want to bring a tick home!

If you live near water, you know about mosquitoes. Did you know that mosquitoes actually feed on fruit and nectar? Most of the time, they glide from flower to tree, just as harmless as butterflies.

Once the female has mated, watch out! Buzz-z-z-z-z. She turns into an attack missile, out for blood. Not for herself, but for her growing eggs. Blood has valuable protein that her eggs need.

A mosquito's proboscis has six fine needles. Some are sharp and used to pierce skin. Others have edges like saw blades. They make the cut bigger. The mosquito sucks the blood up a long tube through her proboscis. Through a second tube, she spits saliva. The saliva has a chemical that keeps the wound from closing.

You can protect yourself from mosquitoes. Get rid of standing water in your yard. Wear long clothing when you are outside at sunrise or sunset. That's when mosquitoes are active. When you hear buzz-z-z-z-z, take aim and slap!

For Americans, mosquitoes are mainly a nuisance. They used to be much worse. You see, mosquitoes can carry disease from person to person. Mosquito-borne diseases include malaria and West Nile virus. Not long ago, malaria was the number-one killer in the United States. Today, though, Americans rarely get malaria.

In Africa and other places, however, malaria remains a huge problem. It kills at least a million of people each year. Health workers are struggling to fight this dread disease.

It's impossible to describe all blood feeders. There are too many. About 14,000 kinds of arthropods are blood feeders, including fleas and some moths. Even some birds, such as the oxpecker and vampire finch, feed on blood.

Don't get creeped out, though. Just use your common sense outdoors. You'll keep your body safe from creatures that are out for blood!

Article by Shirleyann Costigan. Top-of-page photograph by Michael & Patricia Fogden/Minden Pictures/National Geographic Image Collection. "Out for Blood" appears on page 18 of the October 2008 issue.