A man and a woman were today isolated in hospital with the first suspected case of bubonic plague in New York for more than a century.

Though the disease can today usually be treated with antibiotics, it is one of a handful of agents security experts believe could be used in a bioterrorist attack.

The source of the disease may however prove to be several thousand miles away in the couple's native New Mexico, and fleas found close to their home have tested positive for plague.

Health officials said they believe the couple became infected near their home in Santa Fe before they arrived in New York on November 1.

But investigators are also conducting tests in New York as a precaution.

About 10 to 20 people usually get plague each year in the United States, mostly in the western states. One in seven cases is fatal.

The 53-year-old man and 47-year-old woman showed up at a New York hospital on Tuesday, complaining of fever, weakness and swelling. The man is said to be in critical condition, and the woman stable.

While doctors are almost certain the cases are plague, tests to confirm the disease remain incomplete.

The plague cases would be the first in the US this year and the first in New York in more than a century.

Health officials in the city yesterday repeated their assertion that the public is not in danger because bubonic plague cannot be passed from person to person.

In extremely rare cases, bubonic plague can transform into pneumonic plague, a contagious form. But health officials have said that is unlikely to happen in these cases.

Plague outbreaks have killed about 200m people in the past 1,500 years. The Black Death, which hit Europe in the 1340s, wiped out 25m people - a third of the continent's population - and 13m in the Middle East and China.