A virus infecting up to 90 percent of adults worldwide, the JC virus, may have a newly discovered activation pathway affecting 10 percent of adults, Ars Technica reports. This human polyomavirus 2, often acquired early in life via fecal-oral transmission, typically remains dormant in cells for life. For most, it causes no issues.

However, when activated, it can transform into a deadly form causing progressive multifocal leukoencephalopathy (PML), a devastating brain disease. PML attacks myelin sheaths, leading to nerve cell damage, paralysis, and death, symptoms mimicking stroke or MS. First isolated in 1971, PML was rare until HIV/AIDS, where 2-5 percent of patients developed it before HAART treatment in 1996 reduced cases significantly.