At some point prior to the year 2265, during a Starfleet mission, Melander was exposed to a strain of Ametan rubeola. The virus was controlled by regular vaccinations, and remained dormant in his system for years.
In 2265, Melander was assigned to a cultural observation team to planet NGC 667 to study the Grennai race. During this mission, the virus in his system mutated due to NGC 667's atmospheric radiation. The mutated virus spread to the Grennai village under observation, before Melander himself succumbed. (TOS - Constellations short story: "First, Do No Harm")