Halloween got a little weirder this year with news that some people can actually sweat blood. At first there was only one case, discovered by doctors in Florence, Italy, published in the CMAJ on Monday. But since then more cases of sweating blood have been reported to Jacalyn Duffin, a Canadian historian and hematologist, who searched the medical literature for references to this bizarre condition.

‘It seems that humans do sweat blood, albeit far less often literally than metaphorically.’– Jacalyn Duffin, medical historian, Queen’s University

When the Italian physicians first encountered the 21-year-old woman who appeared to sweat blood, they assumed she was faking the condition. But then they diagnosed her with hematohidrosis, a rare disease characterized by the “spontaneous discharge of ‘blood sweat’ through intact skin,” although they still don’t know what causes it or how the blood ends up passing through the skin like sweat.

The alarming image of the unidentified woman’s blood-dripping face triggered a media frenzy, causing the CMAJ to temporarily drop its paywall.

Duffin is intrigued by that reaction. She searched back through the medical literature and found reports about sweating blood that dated back to the third century BC.

“It seems that humans do sweat blood, albeit far less often literally than metaphorically,” she wrote in a commentary accompanying the medical report.

Even though the literature indicates it’s been happening throughout human history, Duffin notes that there is a reluctance to believe in sweating blood, possibly because of its association with “religious mystery.”

Hematohidrosis patient

Jacalyn Duffin, a Canadian historian and hematologist, says five people have reached out to her in recent days to say they’ve experienced sweating blood. (CMAJ)

“Other rare conditions are not viewed with similar skepticism,” she wrote. “Ironically, for an increasingly secular world, the long-standing association of hematohidrosis with religious mystery may make its existence harder to accept.”

Since Monday, Duffin has been contacted by five people who say they’ve experienced the strange occurrence of sweating blood.  Now she’s working with a blood coagulation expert and a dermatologist to further investigate this perplexing condition.