The killer bug, also known as the plague, swept through insurgents training at a forest camp in Algeria, North Africa. It came to light when security forces found a body by a roadside.
The victim was a terrorist in AQLIM (al-Qaeda in the Land of the Islamic Maghreb), the largest and most powerful al-Qaeda group outside the Middle East.
It trains Muslim fighters to kill British and US troops.
Now al-Qaeda chiefs fear the plague has been passed to other terror cells or Taliban fighters in Afghanistan.
One security source said:
"This is the deadliest weapon yet in the war against terror. Most of the terrorists do not have the basic medical supplies needed to treat the disease. It spreads quickly and kills within hours. This will be really worrying al-Qaeda."
Black Death comes in various forms.
Bubonic Plague is spread by bites from infected rat fleas. Symptoms include boils in the groin, neck and armpits. In Pneumonic Plague, airborn bacteria spread like flu.
It can be in the body for more than a week, highly contagious but not revealing tell tale symptoms.
The al-Qaeda epidemic began in the cave hideouts of AQLIM in Tizi Ouzou province, 150km east of the capital Algiers. The group, led by wanted terror boss Abdelmalek Droudkal, was forced to turn its shelters in the Yakouren forest into mass graves and flee.
The extremists supporting madman Osama bin Laden went to Bejaia and Jijel provinces, hoping the plague did not go with them.
A source said:
"The emirs (leaders) fear surviving terrorists will surrender to escape a horrible death."
AQLIM boss Droudkal claims to command around 1,000 insurgents. Training camps are also based in Morocco, Tunisia and Nigeria.
AQLIM bombed the UN headquarters in Algiers in 2007, killing 41. Attacks across Algeria last year killed at least 70 people.
In an interview last July, Droudkal boasted his cell was in constant contact with other al-Qaeda "brothers".
An Al Qaeda affiliate in Algeria closed a base earlier this month after an experiment with unconventional weapons went awry, a senior U.S. intelligence official said Monday.
The official, who spoke on the condition he not be named because of the sensitive nature of the issue, said he could not confirm press reports that the accident killed at least 40 Al Qaeda operatives, but he said the mishap led the militant group to shut down a base in the mountains of Tizi Ouzou province in eastern Algeria.
He said authorities in the first week of January intercepted an urgent communication between the leadership of Al Qaeda in the Land of the Maghreb (AQIM) and Al Qaeda's leadership in the tribal region of Pakistan on the border with Afghanistan. The communication suggested that an area sealed to prevent leakage of a biological or chemical substance had been breached, according to the official."We don't know if this is biological or chemical," the official said.
The story was first reported by the British tabloid the Sun, which said the Al Qaeda operatives died after being infected with a strain of bubonic plague, the disease that killed a third of Europe's population in the 14th century.
But the intelligence official dismissed that claim.