Kirk, Spock and others in “Unfinished Business”

Kirk

Total senility?

Spock

Yes, Captain. In a very short time.

Kirk

What a way to die. All right. I want all the research facilities, all the scientific technicians, to start round-the-clock research immediately. I want the answers, and I want the remedy. Let's start by finding out why Chekov hasn't been affected.

McCoy

I'm doing what I can. [to Spock] You're perfectly healthy.

Spock

[sitting up] I must differ with you, Doctor. I'm having difficulty concentrating, which is most disturbing. My eyesight appears to be failing, and the normal temperature of the ship seems to me to be increasingly cold.

McCoy

I did not say you were not affected, Mr. Spock! You are perfectly healthy...that is, for any normal Vulcan on the high side of a hundred!