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In an open letter to Abu Sayyaf, the terrorist group responsible for executing John Ridsdel, Canadian author Alexandra Sellers says he had tuberculosis and the militants should be worried about having contracted the disease.

Ridsdel was beheaded Monday in the Philippines after a ransom deadline passed. He had been kidnapped six months earlier by the terrorist group.

In the letter posted online, Sellers, a romance novelist who lives in Vancouver, says she was childhood friends with Ridsdel, and reconnected via the internet later in life.

She says many people can attest to Ridsdel’s “kindness and generosity, his sense of adventure, his intelligence, his humanity.”

But she also has a warning for the people who killed him.

She writes “a short while before [Abu Sayyaf] kidnapped him for money, John had been diagnosed with tuberculosis.”

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“Did he warn you that he needed his medicine? Perhaps you didn’t pay attention. You should have,” the letter reads. Tweet This

“Oh, yes, this is a disease that will kill if it is not treated, and how will you get treatment, a wanted man, a hunted killer?”

FULL COVERAGE: John Ridsdel’s kidnapping

According to Health Canada, TB is spread by breathing in the bacteria.

Sellers says the militants, who she calls “Father of the Executioner,” should have cared when Ridsdel coughed or sneezed while the militants “stood over him with guns and machetes.”

“Even if you never came nearer to him than to bathe your hands in his blood when you murdered him, you should care.

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“Because everyone who breathed near him now has the bacteria in their lungs. And sooner or later, one in ten will develop the disease.”

RELATED: Timeline of John Ridsdel’s kidnapping and execution in the Philippines

She claims this mistake made will be the terrorists’ undoing.

“You have the disease now, and it will manifest—oh, soon! You will think it is flu, probably, but you are a dead man.” Tweet This

She also warns that the disease could spread even further to the group members’ families.

“It’s not only yourself that you have killed in killing my friend … You bring death to all you meet now.”

You can read the full letter to Abu Sayyaf on her website.