A 7-year-old girl contracts the bubonic plague and suffers a 107 fever, vomiting, painful buboes, and seizures; a yoga instructor finds out that what she thought was influenza is actually necrotizing fasciitis caused by Streptococcus intermedius and Peptostreptococcus; a state wrestling champion contracts angiostrongyliasis.

A teenage ballerina suffers from painful migraines and muscle failures; a young missionary's skin crawls when he uncovers an unholy revelation.

A mother suffers with abdominal pain and nausea for four years until the true cause of her condition is discovered; a 2-year-old girl develops a mysterious lump on her neck.

A small-town boy's summer fun is cut short by a life-threatening Neisseria meningitidis infection. An oil rig worker and daredevil experiences sensitivity to light, muscle pain and horrifying hallucinations from Trichinella spiralis after eating bear meat. A former beauty queen suffers an extreme case of asthma for one year from a nasal packing contaminated with Aspergillus niger fungi.

A Peace Corps volunteer traveling to Burkina Faso suddenly suffers from excruciating bouts of nausea and diarrhea from Entamoeba histolytica, and must find medical help before it is too late; a young woman almost suffocates due to hantavirus pulmonary syndrome she got from breathing dust while cleaning up contaminated with mouse feces; an 18-month-old boy gets infected with Baylisascaris procyonis worms which cause his brain to swell and have neurological damage, eye fluttering, and unresponsiveness.

A newborn baby develops hydrocephalus from a parasitic disease called toxoplasmosis, which was contracted from his mother; an 11-year-old boy suffers headaches, vomiting, seizures, and starts speaking gibberish from being infected with the eastern equine encephalitis virus; a teenager living on a farm suffers a severe cough and fever from Paragonimus kellicotti.

A Canadian 13-month-old toddler gets breathing difficulties and mild vomiting from accidentally swallowing a button battery (for what the doctors originally thought was a coin); a college freshwoman gets migraines, nausea, on-and-off blindness, hallucination-like partial seizures, repulsive taste and two lesions in her brain from infection with Taenia solium; in 2008, a 6-year-old girl gets a headache, a rash, high fevers, and dies from Rocky Mountain spotted fever.

A 4-year-old boy literally drowns in his own mucus, being a victim of an outbreak of the Enterovirus D68 virus; a 7-year old scrapes his knee on a rock at the beach, but when his father puts a liquid bandage on it, it causes it to swell the size of an orange – then, his mother squeezes the wound and finds a sea snail (Littorina scutulata) inside; a college student experiences loss of appetite, fatigue, fever, jaundice, and repetitive nightmares that turn into a reality from getting infected with malaria caused by the Plasmodium falciparum parasite after returning from a trip to Ghana.

An athlete gets headaches, stomach pains, breathing difficulties, and a liver mass from Entamoeba histolytica from a trip to Nicaragua; a teenager develops a leg welt and then gets fatigue and red spots on his body over the course of several years. When the spots open, the doctors discover he has leprosy from coming into contact with an infected nine-banded armadillo (Dasypus novemcinctus). A woman suffers severe headaches, nausea, vomiting, and seizures from Exserohilum rostratum fungi invading her brain.

During a fishing vacation, a soccer coach goes into septic shock from Vibrio vulnificus necrotizing fasciitis after banging his ankle on a barnacle; a 7-year-old girl gets facial tics, bouts of anxiety, and turns suicidal after a streptococcal brain infection; a makeup artist gets severe stomach cramps and watery diarrhea from Giardia lamblia.

An 11-month-old boy experiences wheezing and vomiting after swallowing a button battery; a landscaper becomes infected with Paragonimus kellicotti; a female boxer gets a severe headache, a 105 fever, and impaired vision from cat scratch fever caused by Bartonella henselae.

A 2-year-old girl experiences severe pain, trouble breathing, and vomiting from a bark scorpion sting; a neuroscientist experiences multi-organ failure and nearly dies from a leptospirosis infection that he caught while swimming in Hawaii; a 20-year-old woman suffers from severe vomiting, extreme headaches, twitching and a strange crawling sensation in her head that turns out to be caused by Balamuthia mandrillaris and dies from it.

A young college student contracts Acanthamoeba keratitis from sleeping in her contact lenses; an infant becomes temporarily paralyzed and almost dies from accidental ingestion of soil that contained the most deadly toxin known to science, botulinum toxin. A new mother has extreme pain from a piece of gauze that was left inside her abdomen during a C-section.

A California teenager nearly suffocates from hantavirus pulmonary syndrome; a Hawaiian personal trainer suffers years of excruciating neuropathic pain from ciguatera poisoning caused by Gambierdiscus toxicus toxins; a Michigan health worker nearly dies from babesiosis.

A 13-year-old boy gets a cut on his leg that nearly leads to amputation when it becomes infected with Staphylococcus aureus, causing necrotizing fasciitis; a couple move into their new home, but over the course of three years, the man suffers a spreading/oozing rash, poor coordination, body aches, breathing difficulties, and weight loss, while his girlfriend suffers memory loss, fatigue, and suicidal depression, both from chronic low grade carbon monoxide poisoning; an 8-year-old girl from Hawaii gets a growing mass in her eye from Toxocara canis.

The third episode of the series to be Christmas-themed – A baby is having problems controlling the left side of her body, along with multiple miniature seizures before doctors discover the right side of her brain is malformed due to a Cytomegalovirus infection; a PR executive suffers a year-long ailment that turns out to be a cyst caused by Echinococcus granulosus tapeworms infecting her spleen; seizures ruin a schoolteacher's Thanksgiving, and when they worsen around Christmas, doctors discover a pork tapeworm cyst.

This episode looks back at three cases from past episodes: a teenage boy with a grill bristle in his intestines (A Deadly Swim); another teenage boy with leprosy (Help! My Son is a Leper); and a teenage girl whose lungs are afflicted with Cryptococcus gattii fungi (My Body Is Rotting).

A infant becomes totally paralyzed after accidental exposure to botulinum toxin; a Texas man develops severe pain in his scrotum for which doctors think is an ingrown hair, but it actually turns out to be botfly larvae which infected him while he urinated outdoors in Costa Rica; a Florida model/dentist is confounded by her mysterious weight gain, and over the course of a year she suffers continuing weight gain, blurry vision, crippling hand pain, and muscle weakness before it is discovered her breast implants are contaminated with Aspergillus fungi.

An Idaho 17-year-old football player is struck with a sore throat, breathing difficulties/lung pain, confusion, kidney failure and even tackles his own mother from Group A streptococcus bacteria infecting his blood; an Illinois mother of three kids develops an itchy eye which leads to face numbness, loss of depth perception and blindness as the result of an Acanthamoeba keratitis infection; a dockmaster develops crippling pain in his hand from a Mycobacterium marinum infection along with pimples on his palm from barnacles growing in his hand.

The show's fourth Christmas-themed episode, and the second episode of the series that looks back at three past cases, including: a baby whose brain was infected with Cytomegalovirus in utero (All I Got For Christmas Is Brain Surgery); a woman whose eye is infected with the worm Dirofilaria immitis (A Holiday in the Hospital); and a man who has a surgical sponge left in his colon (also A Holiday in the Hospital).

A young girl is brought to the hospital with stabbing pains in her scalp; a mother's mysterious symptoms threaten to tear a family apart; a deadly invader turns a relaxing vacation into a nightmare.

A man falls comatose; a college student has a sore throat; and a young woman's symptoms confound experts.

An infant falls mysteriously ill; a biology professor becomes an unexpected host; and a family man is brought to the brink.

A new mother is in critical condition; a young boy has an ailment that no antibiotics can cure; and a young man has an invader in his brain.

A young boy has a strange condition; a woman's symptoms push her to madness; and a toddler falls into a trance-like state when something begins eating his brain.