From the article:
The scientists demonstrated this cancer-like mobility through a technique called parabiosis: surgically attaching two specimens together so they share the same blood supply for several months. […] Normal mice that had been joined to genetically modified partners for a year “contracted” Alzheimer’s disease. Song says the amyloid-beta traveled from the genetically-modified mice to the brains of their normal partners, where it accumulated and began to inflict damage.
The problem is partly due to an increased permissiveness of the blood-brain barrier as we age that allows entry from other parts of the body:
“The blood-brain barrier weakens as we age,” says Song, a Canada Research Chair in Alzheimer’s Disease and the Jack Brown and Family Professor. “That might allow more amyloid beta to infiltrate the brain, supplementing what is produced by the brain itself and accelerating the deterioration.”