By their very nature, parasites — organisms that live on another organism and freeload off them for food and survival — are pretty gross. In some cases, parasites may actually benefit the planet — and possibly even human health. But in other cases, they can be man's worst enemy.

Scientists are making progress fighting these marauding moochers. In October 2015, three scientists received the Nobel Prize for discovering anti-parasite drugs that are expected to improve life for millions of people around the world. The three doctors separately discovered substances in herbs and soil to convert to drugs that combat parasites that cause malaria, river blindness and elephantiasis.

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Unless you’re traveling to a developing country, your risk of encountering the parasites that cause those infections is vanishingly low. But if you’re visiting a remote part of the world, check to see whether there’s a recommended vaccine for any parasitic illnesses found there. And know that a few dangerous parasites are found in the United States, including roundworm and brain-infecting amoebas.

Here is a roundup of five dangerous illnesses you can get from parasites, along with tips for avoiding them from the Centers for Disease Control and Prevention.

Lymphatic filariasis and elephantiasis

W. bancroftiAdults who are old enough may remember witnessing with awe an enormous leg — as big as a tree trunk — floating in a jar at the Smithsonian. The jar was labelled “elephantiasis.” The condition is caused by blockages in the lymph system from a disease caused lymphatic filariasis, which affects nearly 120 million people in tropical and subtropical regions, according to the CDC.

Symptoms. Lymphatic filariasis causes fluid build-up in the body, which over time can lead to elephantiasis, often marked by the severe and disfiguring enlargement of the lower trunk and limbs.

How you get it. Lymphatic filariasis is caused by the parasite Wuchereria bancrofti, a thread-like worm carried by mosquitoes.

How to prevent it. Avoid infested areas by checking the U.S. Department of State’s travel website for warning advisories. If you do travel to an infested region, apply insecticide to exposed skin, wear long pants and sleeves and use a bed net sprayed with insecticide.

But don’t obsess over a mosquito bite or two on your travels. You have to be bitten over a period of months to develop the disease, the CDC notes, so tourists rarely get it. (Photo: CDC/Centers for Disease Control and Prevention)

Human bot fly myiasis

This painful skin infestation occurs in subtropical areas of the Americas, according to the U.S. Army Public Health Command.

Symptoms. Severe itching, shooting pain and bleeding boils and nodules.

How you get it. From maggots (bot fly larvae). The biting flies lay eggs on your skin, and larvae from the eggs can burrow underneath it.

How to prevent it. Avoid traveling with open wounds. Use window screens and mosquito netting, apply insect repellent and wear long clothing and a hat. Also, iron any clothes that have been hung out to dry, as flies may deposit eggs on the fabric. The heat kills the eggs.

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malariaThis disease, which can be mild or severe, is carried by one-celled organisms called protozoans and transmitted by mosquitoes. According to CDC, the microscopic parasite develops inside human red blood cells, where waste substances accumulate and eventually are dumped into the bloodstream. There, they trigger an immune response.

Symptoms. Fever, vomiting, headaches and dry, hot skin followed by drenching sweats. In severe cases malaria leads to coma and death. In Africa, a child dies from malaria every minute. Antimalarial drugs taken for prevention by travelers can delay the appearance of malaria symptoms by weeks or months.

How you get it. In most cases, from parasites transmitted through the sting of a mosquito from the genus Anopheles.

How to prevent it. Take anti-malaria pills if your travel doctor recommends them. Sleep in a screened room inside a bed net sprayed with insecticide. By day, apply insect repellent. (Photo: CDC/Centers for Disease Control and Prevention)

River blindness (onchoceriasis)

This disease infects about 25 million people worldwide, according to the World Health Organization. About 1 percent of cases lead to permanent blindness.

onchoceriasisSymptoms. Itching, rashes, skin discoloration and blindness.

How you get it. From black flies that typically live near rivers in Africa, Yemen and tropical regions of South America. Parasitic worms (nematodes) are spread by the fly’s bite and grow beneath the skin, according to the CDC.

How to prevent it. Although the risk to tourists is low, the CDC advises applying DEET to exposed skin and wearing protective clothing treated with permethrin. (Photo: CDC/Centers for Disease Control and Prevention)

“Brain-eating” infections

Known popularly as a “brain-eating amoeba,” Naegleria fowleri causes a very rare but deadly brain infection called primary amoebic meningoencephalitis. The amoeba is found around the world, but in the United States, most infections occur in Southern-tier states . Only 35 known U.S. infections have occurred from 2005 to 2014.

Symptoms. Headache, stiff neck and vomiting, progressing to confusion, seizures and hallucinations. Coma and death usually occur about five days after the onset of illness, which is 97 percent fatal, according to the CDC.

How you get it. From getting water infested with the parasite up your nose. N. fowleri thrives in warm freshwater such as rivers, lakes, streams, swimming pools without sufficient chlorination and even some tap water. But not to worry: Swallowing water containing N. fowleri is harmless, the CDC advisory notes.

How to prevent it. Check local warnings before lake or river swimming. Avoid diving, jumping or otherwise submerging your head in the water. If you do sinus rinsing, the CDC warns, boil the water first or use a chlorine bleach solution to disinfect it.

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Steve Evans, MA, is an award-winning journalist with more than 20 years experience in daily news, investigative, health and business journalism. Among other jobs, he has served as managing editor of the Central Virginia Newspaper Group, as a senior writer for SNL Financial and as a staff writer for The Progress Index and the Richmond Times-Dispatch.