One of the most basic implementations of genetic manipulation is modifying existing microscopic organisms such as bacteria and viruses. At PL 5, scientists are able to alter existing diseases to be resistant to particular medicines and resilient enough to survive in conditions that would kill their naturally occurring relatives.
At a later stage of PL 5, researchers learn to splice together the DNA of two or more different diseases to create super viruses. At this point, they can tailor such characteristics as the disease's incubation period and primary, secondary, and tertiary symptoms. They can even make it resistant to all known treatments and, at the same time, create a wholly effective remedy-putting them in the position of being able to infect the entire world and only providing the cure to those they deem worthy.
The flip side of this, naturally, is that scientists are able to create synthetic medicines-antibiotics and other drugs-that combat diseases that were previously uncontrollable. Unfortunately, nature is at least as inventive and resourceful as science. Whenever a new cure is developed, it is not long before scientists discover one or more diseases that have natural immunity to it.