Dorothy shrugged. “Landfall station has an airlock, and there are lots of rules to avoid contamination. But with an outside world that looked so safe…” She trailed off. “Look, prions weren’t well understood even when we left. They’re just like this.” She pointed again. “Badly folded proteins. They change the function of the original. They can spread their ‘mutation’ to correct forms. They’re stable, so they tend to bioaccumulate in healthy tissue.