It is believed that the Ringworld Engineers prevented any disease bacteria from inhabiting the Ringworld when it was constructed. Even by the time of the First Ringworld Expedition in 2851, infectious disease was so rare it was unknown in many areas. During that expedition, Louis Wu speculated that a colonic bacteria had mutated into something deadly, causing the downfall of the City Builder civilization. See also superconductor plague.
- Ringworld chs. 16, 17