SCIENCEThe Real Reason Prion Diseases Spread Without DNA8 min read

When Scientists Called It a Slow Virus

The term “slow virus” was coined in the 1950s to describe a group of diseases that behaved unlike anything in the standard infectious disease playbook. Unlike typical viral infections, which produce illness within days or weeks, these diseases incubated for months or even years before symptoms appeared. Scrapie, which caused neurological deterioration in sheep and goats, was the original slow virus candidate. By the 1960s, researchers had applied the label to certain human conditions as well, including Creutzfeldt-Jakob disease, which seemed to cluster within families. The term described a behavior pattern rather than an actual pathogen — a placeholder for something scientists had not yet identified. It allowed medicine to classify the diseases without actually explaining them, which is how the mystery survived well into the 1980s.

Kuru and the Fore Tribe of Papua New Guinea

Among the most striking examples studied during this era was kuru, a fatal brain disease that devastated the Fore people of Papua New Guinea. Researchers traced the spread of the disease to a funerary practice in which tribal members consumed the brain tissue of those who had died from the disease. The pattern of transmission — through the direct consumption of infected neural tissue — pointed strongly toward something in the brain matter itself being the infectious agent. Research in chimpanzees during the 1960s took this further, demonstrating that Creutzfeldt-Jakob disease could be transmitted to the animals by feeding them brain tissue from affected individuals. Under a microscope, the brain tissue of animals and humans affected by kuru, scrapie, or CJD all showed the same characteristic damage: tiny holes throughout the tissue, giving it a sponge-like appearance, which led to the term “spongiform” being applied to this entire class of disease.