Nearly 200 people have been tested for tuberculosis on P.E.I. this summer after a case was diagnosed at a seniors home in Charlottetown in early June.

Dr. Lamont Sweet says tuberculosis can remain dormant for decades. (CBC)

It was the first case of TB on the Island in four years.

Dr. Lamont Sweet, the province's deputy chief health officer, suspects the woman contracted the disease many years ago, and it developed into an active case recently.

"The bacteria remains dormant for many decades and then in later life, as the ability to fight infection begins to decrease, the person's tuberculosis comes back and they go to a state of active tuberculosis," Sweet told CBC News on Tuesday.

Until the mid-20th century, a diagnosis of TB, a bacterial infection that attacks the lungs, was often a death sentence. These days, in wealthier parts of the world, few people get tuberculosis and the disease can be treated with antibiotics.

The Charlottetown woman who was diagnosed with active TB this summer was treated with four kinds of antibiotics, and is no longer contagious. To ensure the disease did not spread, 187 people she was in contact with were tested.

"It's a lot of people. It's a lot of work for public health nursing, to carry out the testing, skin testing, a lot of X-rays to be done," said Sweet.

"It's not uncommon for someone who's in a community-care facility, a long- term-care facility, or in a school setting or a university setting to come in contact with 100 to 200 people."

While skin testing showed fewer than 10 people had been exposed to TB, all the followup X-rays came back negative.

Sweet said those who tested positive likely had been exposed to the bacteria years ago or had the vaccine.

P.E.I. has the lowest rate of tuberculosis in the country.