Warning signs: "You first know you have fibromyalgia when you have trouble sleeping and wake up unrefreshed, with aches and pains in your muscles," says Carolyn Dean, a nationally renowned medical doctor and nutritionist based out of Kihei, Hawaii. Other clues include fatigue for no apparent reason or irritable bowel syndrome resulting from anxiety or stress.
Dealing with it in the workplace: There is no widely accepted cure or treatment. Dean prescribes her patients magnesium baths and oral magnesium citrate powder in water and a yeast-, wheat-, dairy-, and sugar-free diet, but consult your doctor for diagnosis and a treatment plan.
Medical condition: Celiac Disease
What it is: Some people can't process gluten. When they eat something with the protein in it, their immune system attacks the gluten and ends up hurting the small intestine in the process—making it hard for the body to absorb important nutrients.
Warning signs: Are you visiting the bathroom a lot or feeling tired frequently? Scott Mann, a graduate student and environmental educator in Harrisburg, Penn., was diagnosed with celiac disease in 2008, a decade after it became routine for him to have to sprint to the bathroom two or three times a day.
Dealing with it in the workplace: You can't manage it well unless you're eating a gluten-free diet, which is why this is a frustrating condition for many people. Mann spent several years working in information technology as a field technician before he learned he had Celiac Disease. "It wasn't all that bad when I worked in a regular office environment," he says. "I'd go down the hallway to the bathroom. But when I was out in the field, it became a problem." However, considering he'd sometimes be behind the stall for a half hour, it was a dilemma throughout his IT career.
Medical condition: Vasovagal Syncope
What it is: It's a condition in which a person's heart rate and blood pressure suddenly drops. The blood flow to the brain is reduced, which causes a brief fainting spell. It isn't life-threatening, but you wouldn't want to pass out while driving.
Warning signs: Fainting a lot? Feel tired and rundown all the time? Last October, Laura Grover, 47, of Raleigh, N.C., and an executive at Quintiles, a contract research firm, was asked by colleagues if she felt OK. Since Grover was used to feeling sick, she waved off her coworkers and went to a meeting. But a few minutes later, she couldn't focus. Her head felt heavy. She was hot and cold simultaneously, nauseous, and her legs shook violently. She then checked into the emergency room. In November, after more episodes and doctor visits, she was diagnosed with vasovagal syncope. She is still working on finding a suitable treatment.
Dealing with it in the workplace: "Be honest, early," says Grover. "It's hard to hide something that can [surface] at work."
[In Pictures: 7 Work Habits That Are Making You Sick.]
Besides, your coworkers may help you. "Standing for a long time can cause an episode, so [my colleagues] help ensure I'm safely seated, even when I'm in a crowded space," says Grover, who adds that many professionals don't seek out a doctor because they assume their medical problems are stress-related. That chain of thinking is arguably a universal trait of human nature. Says Grover: "It seems people want it to be stress."
Corrected on 03/04/2013: A previous version of this story misspelled the names of two medical conditions: Abductor spasmodic dysphonia (ABSD) and Vasovagal Syncope.