Hidradenitis suppurativa is a disease of the apocrine sweat glands, found in the armpits and the groin.

The illness also contributed to Marx's poverty, Professor Shuster says. "This new diagnosis is not just important in terms of historical accuracy," he said. "The skin is an organ of communication, which is why its disorders produce so much psychological distress, with depression of self-image, mood and wellbeing, and with self-loathing and disgust.

"In addition to reducing his ability to work, which contributed to his depressing poverty, hidradenitis greatly reduced his self-esteem. This explains his self-loathing and alienation, a response reflected by the alienation Marx developed in his writing."