Ailments

The reality of the future is that there's better science, technology, and practices to keep people healthy. The problem is that there's more ways to get sick in the future than there ever was in the past. New planets, new environments, and the natural (and unnatural) mutations of existing viruses and bacteria make things tough at times. Medical services aren't necessarily evenly distributed either, meaning immediate care isn't always available. Worse, closed environments like O'Neill Cylinders, habitats in hostile environments, and spacecraft, communicable diseases can result in devastating events.