Facial legions were a symptom of the plague

The Red Plague was a plague caused by a virus which killed more than one million people on Vulcan during or near the 2070s. Symptoms include tiredness, paranoia, and, when well-progressed, blemishes on the face and hands. Nectar from the Shin-Ka-Ti plant provided the only known cure.

The plague erupted from the out-region of Kendha-Ya on Vulcan, but the virus itself may not have originated there. An outbreak occurred on Circe-Two in 2279, resulting in an epidemic that killed nearly all of the 30,000 colonists there in less than two months. Vulcans were more susceptible to the virus than humans. When exposed to the virus in 2279, Captain Spock felt faint in less than one minute. Leonard McCoy developed paranoid symptoms fairly quickly, resulting in him fleeing the USS Enterprise rather than reporting that he was exposed. (TOS comic: "Terminally Yours")

Spock said the plague on Vulcan occurred two hundred years prior to the events of TOS comic: "Terminally Yours".
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