Researchers have found that a group of viruses that cause severe stomach illness — including the one famous for widespread outbreaks on cruise ships — get transmitted to humans through membrane-cloaked ‘virus clusters’ that exacerbate the spread and severity of disease. Previously, it was believed that these viruses only spread through individual virus particles. The discovery of these clusters, the scientists say, marks a turning point in the understanding of how these viruses spread and why they are so infectious.