A young mother expecting her second child suddenly comes down with a stuffy nose and itchy throat. They appear to be typical flu symptoms, but she still feels feverish and soon starts coughing. Her breathing deteriorates to the point where she cant' breathe on her own anymore. It turns out that the young woman has developed an Acinetobacter infection. An energetic college student has been healthy, until she suddenly feels like she can't get enough oxygen and starts wheezing. She develops bouts of coughing so bad that she even coughs up worms. The student learns that she has been infected by the Ascaris worm the whole time. A teenage girl living in the countryside suddenly gets headaches. At first, it appears to be from stress, but she soon becomes irritable and develops triple vision. Worse, she even collapses and experiences a seizure. Only when it's too late that it becomes clear that the teenager has had rabies.

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 little girl's health begins to deteriorate when she suddenly comes down with a fever. She then develops reddish spots on her body, stops breathing on her own, and even suffers kidney and liver failure. A spinal tap reveals that the little girl has meningococcal disease. Unfortunately, the disease causes too much damage that the doctors have to amputate her left arm. A home contractor has been relatively healthy, until he suddenly begins to gain weight in his abdominal area, which turns out to be caused by a large tumor. After removal of the tumor, the man feels excruciating pain that spreads from his abdomen to his lower back and chest. Initially, it appears to be part of his recovery process, but an x-ray shows that a retractor has been left in his body after the surgery. 7-year old Kyle Lewis's life turns into a living nightmare when he starts complaining of headaches, and starts vomiting.

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 young aspiring artist suddenly comes down with flu-like symptoms and a swollen face. To make matters worse, his eyes are yellow, he has trouble breathing, and one of his eyes protrude strangely. It turns out that he has Lemierre's syndrome. An active girl's life is put on hold when she starts feeling fatigue and out of breath. Later, she also starts experiencing extreme joint pain, and she gets diagnosed with Lyme's disease. However, further testing shows that the girl has Babesiosis, in addition to chronic Lyme's disease. An active middle-aged woman becomes concerned when she suddenly feels lumps in her chest. After getting them removed, she then starts experiencing bleeding from her right breast. After getting that taken care of, though, she feels a growing lump in her right breast, which turns out to be two surgical sponges.

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 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.

A 1-year old boy who survived a premature birth starts resisting food, suffering a fever, and coughing up blood. After being rushed to the emergency room, the doctors find out that the cause was the boy swallowing a button battery, which burned a hole in his esophagus. A 21-year-old college student from Canada starts suffering a headache that lasts longer than it should. And soon, she starts getting hallucinations, her head starts twitching, she forgets what her age is, and she even starts having grand mal seizures. While resting in the preparation room for brain surgery, she suddenly gets up and yells, then passes out. Eventually, it is discovered that she has an abscess in her brain that caused an infection caused by Streptococcus milleri, which was caused by getting her tongue pierced. A mother/wife starts to get a headache, and soon she also experiences fever, body aches, dizziness, and foul-smelled urine. Her doctor soon discovers that she has an infection called Dengue fever.

A teenage girl has been enjoying her life of gymnastics and nature, when she suddenly receives a scratch on her hand. The problem seems minor, but the scratch fails to heal and becomes several lesions, which also spread to her eyes. The infection appears to be caused by the MRSA bacteria, but it turns out that it is actually caused by Mycobacterium marinum. A middle-aged man suddenly feels as if he's choking and coughs hard. Initially, it appears to be a case of pneumonia, but after a surgery to fix his spine, he develops stabbing pain in his throat. The coughing returns, and it turns out that he did, indeed, swallow a can tab. A young man suddenly starts complaining of itchiness in his body. He develops a headache, nausea, body aches, and an inability to urinate. Tests show that the man has developed meningitis from rat lungworm disease.

A middle school teacher has been having a healthy pregnancy until suddenly, she starts experiencing abdominal cramps and diarrhea. It initially appears to be a case of the flu, but she soon starts bleeding during episodes. At this point, the young woman appears to have hemorrhoids, which progressively worsen. Tests show that she has been infected by the Cryptosporidium parasite. A retired schoolteacher enjoys his retirement with his wife, when suddenly, he feels a lack of appetite for his favorite food. It doesn't appear to be an issue at first, but his appetite continues to decline, and he starts coughing frequently. His coughing fits worsens to the point where breathing becomes incredibly difficult. It appears to be pneumonia, but it turns out that a pea has been lodged in his lung. A young truck driver has been fairly healthy, when suddenly, discovers a strange speck in his vision. Later, he develops pain, irritation, and blurry vision in his right eye.

The second episode of the series to be Christmas-themed – a saleswoman gets a stabbing headaches and a constant burning sensation in her right eye from a Dirofilaria immitis worm that is in her eye; a store manager gets stomach pains, fatigue, nausea, vomiting and even a constantly moving stomach from a surgical sponge being left and forming a hole in his colon; a grandmother working in the forest service gets abdomen pains, fatigue, and pneumonia from Strongyloides stercoralis worms invading her intestines.

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 war veteran suffers fatigue, high fevers, night sweats, strange rashes, bloody urine and jaundice from babesiosis; a 15-month-old baby girl suffers fever, dry mouth, vomiting and bloody diarrhea from E. coli O157:H7, whom she got from drinking raw milk; a biologist suffers extreme fatigue, back pains, anxiety, and vertigo from Aspergillus-contaminated breast implants

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 man has something living in his eye; a young girl mysteriously falls ill; and a woman's bizarre symptoms push her to the brink of insanity.

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 gets coughing, shoulder pains, dizziness, and nausea from Cryptococcus gattii fungi invading her lungs; a saleswoman gets extreme pain in her breast from a brown recluse spider (Loxosceles reclusa) bite; a religious youth organizer gets swelling of multiple parts of the body over the course of three years from the Loa loa worm.

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 teenage girl begins having strange mood swings from anti-NMDA receptor encephalitis; an elderly mother gets chills, hand numbness, wrist pain, and oozing blisters from Vibrio vulnificus she got from being pricked by a crab while making a stew; an athletic military veteran from the Afghan war competing in reality shows returns home from Fiji feeling hotness, a pinching sensation in her arms and toes, and hallucinations from rat lungworm she got from eating a slug in a reality show in the jungle

A Canadian science-lover and traveler suffers a painful lesion caused by botfly larvae; a video game programmer gets suffers from constant vomiting and coughing. When doctors find a large mass in his chest, it is discovered that he has histoplasmosis, which was contracted from inhaling bat droppings in a cave; a 15-year-old girl gets an extremely sore throat, a headache, nausea, and body pains from Lemierre's syndrome.

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.

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 contractor gets hit in the head with a tool, and later starts getting a pounding headache, dizziness, spotty vision, and nausea from having a nail punctured into his brain; a football player in college gets extreme headaches, a stuffy nose, and blurred vision from Aspergillus fungi entering his brain; in 2012, an 8-year-old boy suffers constant vomiting, fatigue, seizures, and eventually dies from amoebic meningitis after being infected with Naegleria fowleri.

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.