Ancient viruses, whose DNA has hitchhiked within the human genome for millennia, may be the cause of many age-related conditions. Scientists have proven for the first time that they can use this viral DNA -- known as'retroelements' -- to predict the age of human cells with'high accuracy.' In recent years, this seemingly inactive'junk' DNA from retroelements has been linked to everything from sleep patterns and memory formation to bipolar disorder. Armed with their new ability to track a person's age via this ancient viral DNA, the scientists now plan to investigate whether new antiviral treatments could reverse the conditions of aging, by deactivating the worst of these viral'retroelement' genes. The new research harnesses previously unknown features of this ancient viral DNA, creating a biological clock to track a person's age from the DNA's chemical changes.