After the scene where Spock is weeping, Leonard Nimoy's fan mail increased exponentially. Viewers were enthralled with the idea that Spock was secretly a reservoir of love and passion instead of an empty emotional void. This reaction inspired further scripts which explored Spock's inner makeup.
The budget-strapped show often made good use of the creativity of its prop staff in coming up with low-cost solutions to otherwise pricey items. Here, the "thermal suits" worn by Spock and Tormolen on the planet's surface were fashioned from 1960s art deco style shower curtains.
This is the only TOS episode in which the three primary female crew members - Uhura, Chapel, and Rand - appeared together. The characters did not appear together again until Star Trek: The Motion Picture (1979).
This is the first episode (in broadcast order) to feature the Vulcan Nerve Pinch.
While under the influence of the virus, Nurse Chapel attempts to seduce Spock. This would be the first depiction of what many fans perceived as underlying romantic tensions between the characters, or at least Chapel's unrequited romantic attraction to Spock.