Sometime prior to treaty negotiations between the United Federation of Planets and Gideon, Captain James T. Kirk had nearly died from the disease. When the prime minister of Gideon learned about Kirk's bout with the disease, this motivated them to request that Kirk be the delegation to their planet in 2268, as he now carried a fatal illness that they had no immunity to.
The plan was for several volunteers from Gideon to be deliberately infected with this disease which would have been sourced from Kirk, with the aim of decreasing their population through death. Ultimately, Kirk passed it on to Odona, who suffered for some time before finally being treated by Doctor Leonard McCoy aboard the Enterprise. She was then to be the one who spread it among the larger Gideon population. (TOS: "The Mark of Gideon")