A very active 41-year-old bodybuilder realizes that a strange lump has grown in her jaw while training for a competition; a boy has infrequent bowel movements.

A woman starts failing after a family vacation; another woman, who has suffered from stomach pains for years, has problems with swelling during menstruation.

A baby develops a mysterious set of symptoms, including a bulging forehead and eye color changes. A teen suffers severe hand pain during swim practice.

A young woman experiences bizarre episodes where they sleep for almost the entire day after a car accident causes them to develop Kleine-Levin syndrome. An 11-year-old suffers a terrible headache.

A young student begins to suffer from paralyzing pain in his back. A couple adopts a girl and they find a huge and swollen lump on her shoulder. Initially suspected to be severe neurofibromatosis, the efforts of doctors to relieve the child's condition by conducting an amputation of the affected arm only makes things worse when bone begins overrunning the site of the amputation, revealing her condition to be the lethal and progressive disease fibrodysplasia ossificans progressiva.

An 18-month-old baby suddenly starts screaming one afternoon in the pool, and begins exhibiting extremely painful symptoms when exposed to sunlight, with porphyria later found to be the culprit. A mother of two children begins to suffer a series of very debilitating headaches.

Rachele Lacount and Mike West are delighted that their baby has started to walk and talk, but suddenly they realize that something is wrong with his right eye. Since middle school, Cherryl Marshall struggled with taming her obscene body odor.

Parents notice their 7-month-old son has troubling physical impairments, then begins violently harming himself, culminating in a horrifying incident where the child bites off his own lower lip; he is later diagnosed with the genetic condition Lesch-Nyhan syndrome. A woman traveling the world starts experiencing unusual back pain and tingling in her feet that rapidly develops into a spreading paralysis; following the successful use of plasmapheresis, the cause of the dangerous condition is identified as Devic's disease, an inflammation of the spinal cord.

Bob was an outstanding middle school teacher who never got sick a day in his life until he was stuck by night sweats, headaches, light sensitivity, and blindness in his left eye. Diane and Howard have a baby boy with a mystery illness. Eventually, another doctor sees the newborn and diagnoses a rare and fatal disease, seen less than 100 times.

Holly is overjoyed at the birth of her second baby, Hannah Hannum, but soon realizes that she has Down syndrome; Carly and Nathan have just gotten married, when Carly falls extremely ill with strong nausea.

Doctors try to understand diseases that are often misdiagnosed, including a case of a newborn who has a cleft in the chest, and is later diagnosed with Marfan syndrome after they begin exhibiting the telltale signs of the condition as they grow older, and another who shows very worrying symptoms.

Marylyn and Randall House become alarmed when their young daughter's motor skills begin to deteriorate; Amanda Gray experiences extreme fatigue after the birth of her second daughter.

After passing out during a soccer game, Allie Isaacson suffers excruciating pain in her legs; Molly Bigford experiences a miscarriage with her first pregnancy and high blood pressure with her second.