Omega IV virus
The Omega IV virus was a virus created on the planet Omega IV as a result of a biological war conducted there hundreds of years before the 23rd century. Eventually, the virus inhabited every aspect of the planet's biosphere, however, over time, nature built up natural immunizing agents in the food, water, and soil to counterbalanced it, causing the indigenous population of Yangs and Kohms to be naturally immune.

The virus was first discovered by the Federation in 2268 when the starship  began a survey of the planet. Captain Ronald Tracey was interested in discovering why the planet's population possessed such a long life span, and beamed down with a landing party in an attempt to find out why. Thinking he had discovered a veritable "fountain of youth", he established relations with the Kohms while the rest of the landing party returned to the ship. A short time later, the entire crew of the Exeter was dead or dying.

The virus had been brought aboard by the landing party, who had not yet developed the natural immunity to the virus, and caused it to spread quickly throughout the ship. It disrupted the body at the molecular level, separating water molecules from other organic compounds until there was nothing of the body left but dehydrated white crystals within a Starfleet uniform.

When the crew of the  came across the Exeter, they found uniforms filled with the grisly crystals and a log entry made by chief medical officer Dr. , shortly before he died, warning them of the illness and imploring them to beam down to the planet immediately. Planet side, Tracey explained to James T. Kirk and his landing party that Omega IV's atmosphere appeared to provide the only immunity to the virus, effectively marooning them on Omega IV. 

Dr. Leonard McCoy, through subsequent research, found this information to be incorrect. Only brief exposure to the planet's atmosphere was necessary to endow immunity; the real tragedy was that if Exeters landing party had only spent a little more time on the planet's surface, they would not have affected the rest of the crew on returning. The indigenous population had long since built up immunity to the deadly virus utilized in the planet's earlier wars; people only lived longer on the planet because it was natural for them to. ()
Microorganisms