A newly engaged Illinois man mysteriously collapses and suffers organ failure and gangrene from Legionnaire's disease. His fiance gazes in horror as he must choose between life and limb; a perky animal-loving eight-year-old girl survives rabies – a disease with a 99% fatality rate; a college freshmen almost goes blind from a bout of Acanthamoeba keratitis.

A special episode with three cases that happened during the holidays – A newlywed woman gets a constant pain in her scalp from botfly larvae; a Virginia man gets sharp back pains and develops a hole in his heart from Streptococcus viridans; a female business consultant and part-time photographer gets valley fever.

A newlywed woman gets extreme headaches, memory problems, sinus pressure, and painful rashes, while her husband gets endless nosebleeds, breathing problems, and brain fog, both from Aspergillus; a councilman loses his spleen to visceral leishmaniasis; a 6-year-old gets a fever, red spots, a swollen mouth, loss of appetite, leg weakness, and breathing difficulties from rat bite fever caused by Streptobacillus moniliformis.

A teenage boy develops stomach pains, starts vomiting blood and fecal matter, and has nausea from accidentally swallowing a bristle from a grill brush, which got lodged in his intestines; in 2011, a 16-year-old girl who swam in a Florida river experiences a headache, fever, vomiting, mental confusion, unusual behavior and dies from amoebic meningitis; an industrial painter has difficulties breathing and walking from Blastomyces dermatitidis.

A teenager caught in the 2011 Joplin tornado in Missouri is recovering in the hospital when he suddenly suffers from a life-threatening case of zygomycosis. A couple traveling who visited South Africa get maggots under their skin; a man suffers coughing fits, severe headaches, vertigo, and violent episodes (including chasing his son with a wrench and punching him in the face) before doctors discover the fungus Cryptococcus neoformans has infected his brain.

These killers lie dormant, waiting for the right time to strike. Eleven-month-old Garrett Richardson ingests the eggs of a worm, and within days the creature travels to his brain and goes about feeding on its tissue. A Vietnam veteran survived the enemy attacks on the front lines, but back at home — unknown to him — a killer is stealthily working its way through his body, laying waste to his organs and lymph system. By the time the medical evidence reveals the source many years later, the killer is long gone. And John Figge and many of his family members live for six years with unwanted hitchhikers from Africa traveling around in their bodies. Only when one of these creatures begins to attack John's brain are they discovered. Can the family get rid of these deadly parasites before the damage is critical? These are all stealthy predators that get inside their hosts, yet work on very different timetables. Dangerous and often hard to detect, this is the story of the war on sleeper cell parasites.

Across the United States, doctors and scientists are fighting a war against some of nature's deadliest creatures — parasites. It is a race against time when a mysterious organism strikes a mid-western city causing the worst parasitic outbreak in the history of the United States. Before the culprit is contained, 100 innocent people have died. In Chicago, doctors must track down a hidden killer that attacks a group of students on a spring break trip, and a flesh-eating monster is on the loose in a Texas community.

Perfecting the art of reproduction have made parasites the most successful life form on the planet. There are some who are violent, manipulative and have taken sex to the extreme. These parasites are sex maniacs. A New Jersey woman finds her apartment infested with one of the most ubiquitous parasites on earth — bed bugs. Part of the reason for their success is the males' aggressive approach to sex, a method known as "traumatic insemination." A cyclist from Colorado discovers that the insect bites on his head are from botflies. In Oklahoma, a chemical sprayer's cough turns out to be the result of parasitic worms that multiply in the human lung.

Parasites are ruthless, single-minded and above all devious. To become the most prolific life forms in the world takes a degree of cunning that others can only marvel at. Meet the most deceptive parasites on the planet, the "Masters of Disguise." Teenager Mallorie Greiner has Acanthamoeba keratitis, a parasite that's intent on eating her cornea. Doctors are bewildered at first because her symptoms look just like herpes. Runner Rick DiMichele is struck down with a debilitating disease. Initially, doctors think it's cancer, then malaria; eventually, they discover he has a tick-borne disease called Babesiosis. And not until he's staring death in the face do Mohammed Meah's doctors find the real cause of his problem: a parasitic worm called Strongyloides that's been lurking undetected in his small intestine and living off the steroids he was taking to combat his colitis.

Nature is a war zone and parasites are the ultimate fighters. They are hungry, they are ruthless and they don't play by the rules. Parasites have evolved countless ways of hijacking their hosts' bodies: some steal food from inside the gut, others lurk in the blood sapping their host's nutrients and some even target their host's brain. These are some of the most horrific cases of parasitic infection in the United States. In Virginia a vicious creature is on the rampage, its next victim is an innocent child. In Michigan a brutal killer has invaded a traveler's blood, and a summer's day at a Florida lake turns into a nightmare when a ruthless monster attacks a young boy's brain.

In Virginia a young woman is attacked by an invincible brain-eating monster that threatens to take her life. A Midwestern farmer does battle with a beast that cannot be killed and a New Yorker is infected with the deadliest parasite on the planet. Three parasites that are almost impossible to destroy; three victims, fighting for their lives. If doctors cannot cure them their only hope of survival is by living with the enemy inside them.

A fit and active dance instructor is suddenly stopped in her tracks by unbearable abdominal pain and vomiting, and a mysterious mass in her colon is revealed to have decaying worms. A middle-aged banker gets seizures from a parasite he picked up from traveling to his home country, Haiti. A scientist finds worms from Africa that threaten to damage his sight living in his skin and eyes.

A schoolteacher deals with an amoeba that eats at his liver and causes him high fever, abdominal pain, sweating, and chills. A young dancer gets worms from eating contaminated pork in her muscle tissue, causing her to get hives all over her body and her limbs to become stiff and painful. A retiree gets scabies from mites he got from staying in the hospital in the past.

A nine-year-old hockey player comes down with encephalitis from the Acanthamoeba, which causes him to lose his concentration and balance, almost killing him. A computer programmer gets severe malaria from the Plasmodium parasite that makes him lose control of his mind and post-malarial neurological symptoms two weeks after an apparent recovery. A marathon runner deals with Leishmaniasis, which causes his spleen to swell.

The Crytosporidium parasite infects a little girl, causing her severe vomiting and diarrhea. A microscopic amoeba kills a firefighter with an autoimmune disorder by causing him meningitis. A rare, vicious tapeworm detaches the retina in a teenaged girl, destroying vision in her left eye.

A cat scratch leads to a horrifying result for a veterinary volunteer; strange liquid oozes from a college student's arm.

Stories of parasites that reside inside their traveling hosts; an Iraq war veteran pulls a foot-long worm from his body.

Daily stomach cramps reveal some of the most common parasites in nature.

A man tries hookworms to cure his allergies; a bloodsuckers gets into a writer's nose.

A middle-aged man who has been caring for his ailing mother gets Babesiosis from walking barefoot to the mailbox. Since he lost his spleen from cancer, he gets severe diarrhea, in addition to fever, fatigue, and night sweats. An active, busy middle-aged woman experiences stomach cramps, explosive diarrhea, lightheadedness, and blurred vision from the Toxocara parasite, which she probably got from contaminated food. An older mother of five children deals with nausea, loss of appetite, muscle spasms, and vomiting from the Strongyloides worm, which she may have picked up from being in contact with infected soil. Because of steroids she took for her breathing problems, her body became more vulnerable to the parasite.

A seven-year-old girl suffers from seizures from a pork tapeworm after being adopted at the age of three from Nepal. A forty-five-year-old machinist deals with blurry and double vision, leaving him legally blind in his left eye, from a raccoon roundworm that he picked up from his turkey hunting. A forty-one-year-old anaethesiologists, along with twenty-eight other anaethesiologists from two different counties, suffer from stomach cramps and crippling diarrhea from the Cyclospora parasite after eating raspberries infected with human feces.

A toddler starts worrying her parents when she loses her appetite for the foods she normally eats, but starts craving salt and even cat food. After developing a rash, her parents find out that she has been infected by the Toxocara canis parasite. A middle-aged executive of a company starts to feel pain in his abdomen, not knowing that he has been infected by a bacteria called ESBL E. coli. A married couple start to feel flu-like symptoms, which turns out they've been infected by the plague bacterium. However, the husband's condition is much worse, and even has to get his calves amputated.

A teenage boy starts complaining of nasal pain and fever after a swim in the lake. It eventually turns out that he has been infected by a rare but flesh-eating bacteria called Chromobacterium. A young actress starts to get mastitis while breastfeeding her baby. A teenage college student gets infected by a fungus called Blastomyces.

A man suddenly grows a pimple that seems minor at first, but eventually oozes and makes his face swollen. A young man that works at a homeless shelter suddenly starts to feel a sharp pain in his back. At first is seems like a simple back injury, but it eventually gets so sharp it spreads to arms and legs. A school-aged boy starts to get a headache after a visit to a friend's house. It doesn't seem too bad at first, but he eventually develops a fever and lays unconscious. It later turns out he is suffering from a condition called Valley Fever.

A fairly young part-time student starts feeling a throbbing headache, which causes her to have the flu and then meningitis. When she starts hallucinating, it's clear she has a teratoma on her ovary that contains brain cells. An eight-year-old boy starts getting a pain in his stomach and vomiting. It seems that the cause is food poisoning at first, but he soon starts to feel intense pain and swelling in his legs, leading to the discovery that he is suffering from a disease where the flu virus attacks the muscle cells. A self-employed plumber starts to vomit just after starting to work night shifts. He tries to hide the condition, but he soon starts misspelling words.