During the final war, many weapons released on both sides carried genetically engineered viruses and tailor made diseases. Nearly all of these diseases have disappeared, as when the hosts died off, the disease had no way of surviving. However, the radiation released during the nuclear explosions caused many of these tailor made horrors to mutate, allowing them to survive, using animals, plants and so forth as hosts to survive. In many ruins and hidden sections of the devastated country, they lie in wait, patient that someday a suitable host will arrive, allowing the disease to spread, wreaking havoc across the globe.
The radiation also caused many common diseases to mutate. A prime example is the bubonic plague. A strain encountered by a wagon train while traveling through New Mexico was so virulent that it reached it's terminal phase within forty eight hours. As mentioned above, many people in the Deathlands are very hardy, and resilient to disease. However, due to mutation some diseases have cropped up that no one has immunity to, making sick even the most healthy of individuals.