Until recently, researchers thought that multidrug-resistant tuberculosis, or MDR TB, was as bad as the disease got.
They were wrong.
In 2005, doctors in South Africa discovered a new strain of the disease. Extremely drug-resistant tuberculosis - or XDR TB - was found to withstand all of the front-line drugs in the tuberculosis treatment arsenal.
Officials in Canada and the United States are searching for passengers who may have travelled with a U.S. man infected with XDR TB on two transatlantic flights this month.
The man flew to Paris on May 12 en route to his wedding in Greece, and then returned home via Prague and Montreal on May 24 before driving into the United States to avoid no-fly restrictions imposed on him during his honeymoon.
MDR TB is resistant to at least two varieties of front-line antibiotics, while XDR strains are resistant to all of them.
Doctors turn to a number of secondary antibiotics that can be used to treat XDR and MDR TB. Most of those drugs are more expensive than the front-line varieties and often have greater side effects.
"It's just more difficult to treat, given the extent of the resistance," said Dr. Andrew Simor, head of microbiology and infectious diseases at Sunnybrook Health Sciences Centre in Toronto.
This kind of antibiotic-resistant tuberculosis is extremely rare in Canada.
Although he has yet to treat someone infected with XDR TB - there have only been two reported cases in Canada, and both times the patient contracted the disease outside the country - Dr. Simor said that treatment would probably last beyond the 18 months required for most MDR patients.
Treatments can last from six months to more than a year for tuberculosis that responds to front-line drugs.
Dr. Simor said that XDR TB could be treatable if detected at an early stage.
"There just aren't good studies to guide us as to how long [patients with XDR TB]should be treated," he said. "You'd treat them ... until there was no evidence of ongoing disease, and then you'd still treat for six months after that."
Tuberculosis remains one of the most common diseases in the world and causes more than two million deaths each year, mostly in the developing world.
Caused by the bacterium Mycobacterium tuberculosis, the disease infects about one in three people in the world, although not all of them exhibit symptoms, experts say.
Treatment-resistant tuberculosis arises when patients are prescribed the wrong antibiotics or fail to complete their prescriptions. In these cases, the bacteria can become resistant to antibiotics, causing the MDR and XDR strains.
Health-care workers in Canada are required to immediately report any diagnoses of tuberculosis to public-health officials, who follow a series of protocols designed to limit the spread of the disease, according to Dr. Howard Njoo, director-general of the Public Health Agency of Canada's centre for emergency preparedness and response.
However, "there's nothing to suggest that XDR is neither more nor less contagious than other strands of tuberculosis," Dr. Simor said.
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Retro diseases
MEASLES
Symptoms: It begins with a fever, runny nose and cough, and develops into a rash around the face, chest and limbs.
How it's spread: Sneezing and coughing.
Dangers: One in 1,000 people infected may develop brain inflammation, which could lead to convulsions, deafness or brain damage.
MUMPS
Symptoms: Fevers, headaches and swelling of the salivary glands. (One-third of people infected don't experience swollen salivary glands.)
How it's spread: Coughing, sneezing, kissing and sharing food, drinks and cigarettes.
Dangers: One in four men experience swollen testicles and one in 20 women face swollen ovaries, rarely resulting in permanent damage or sterility.
TUBERCULOSIS
Symptoms: Loss of appetite, weight loss, fatigue, fever or night sweats. TB in the lungs may produce chest pains, shortness of breath or a cough.
How it's spread: TB bacteria are spread through the air by coughing or sneezing. To become infected, you have to spend several hours a day with an infected person. Someone with TB infection is at the highest risk of developing the disease within the first two years.
Dangers: The fatal disease damages the lungs and other parts of the body, including lymph nodes, kidneys, urinary tract and bones.
The BC Centre for Disease Control, the Public Health Agency of Canada