What You Need to Know About Extensively Drug-Resistant Tuberculosis Why extensively drug-resistant tuberculosis worries health experts.

 -- A woman with extensively drug-resistant tuberculosis (XDR TB) is currently being treated at the National Institute of Health as officials race to find anyone she might have exposed to the dangerous bacteria.

The patient has been in the U.S. for approximately six to seven weeks, traveling through three states — Illinois, Missouri and Tennessee — before being diagnosed, according to the U.S. Centers for Disease Control and Prevention.

The woman reportedly sought treatment for TB several times in India, but did not complete her treatment regimen, which can last months.

ABC News Chief Health and Medical Editor Dr. Richard Besser said not finishing a course of treatment is a common way this dangerous kind of TB can develop.

"This is a big deal and it’s not because this type of TB is more contagious but if you get this, 70 percent of the time it is fatal," said Besser.

Extensively drug-resistant tuberculosis is a condition where TB bacteria have changed enough to circumvent the two most potent antibiotic medicines, as well as most others, according to the NIH website.

Once this form of TB develops, doctors are left with fewer treatment options to battle the disease.

The unnamed patient is currently being treated by Dr. Steven Holland, the chief of the National Institute of Allergy and Infectious Diseases, according to a statement from the NIH.

“The patient will likely receive drugs with activity against TB that he/she has not previously received,” NIH officials said in a statement. “We will confer with other XDR-TB experts, including those at CDC, to determine an optimal regimen.”

Dr. William Schaffner, an infectious disease expert at Vanderbilt University Medical School, said the arrival of XDR TB is "serious business."

"We have to use combinations of drugs that are used less frequently, they may have more side effects and their effectiveness against TB bacteria is not quite as good or less certain," Schaffner said of treating patients with this rarer form of TB.

This extreme form of TB is not more contagious but can be spread through prolonged close contact. Schaffner explained that a person can be more or less infectious depending on how the disease symptoms present.

“If the edges of your vocal chords are infected with TB you really aerosolize lots of TB bacteria,” said Schaffner, who said that happened in rare circumstances.

Tuberculosis can spread through coughs, sneezes, shouts or even singing, according to the CDC. Because it takes time for the bacteria to grow in the blood, health officials may test people multiple times to determine if they have developed the disease.

While there is a TB vaccine, it is not generally used in the U.S. due to its “limited effectiveness for preventing the most common forms of TB and in preventing TB in adults,” according to the CDC.

"The CDC will be figuring out who all those exposed people are,” said Schaffner. “And put them on a spectrum of likelihood of risk from trivial to much more serious."

Cases of XDR TB are extremely rare in the U.S. and the NIH has admitted just about 20 patients with severe or resistant TB in the last 20 years.

Symptoms of the disease include weakness, weight loss, fever and night sweats. As the bacteria affect the lung patients may cough, have chest pain or cough up blood.