It’s established that the substance causing people to lose their inhibitions is spread by touch. Tormolen contracts it on the mission, Sulu and Riley get it from him while they try to prevent his suicide, Riley gives it to Nurse Chapel, etc.
But there’s one case with skin-to-skin contact where the other person doesn’t, IIRC, show any symptoms. Uhura pushes Sulu away from her (“Sorry, neither!”) and yet doesn’t seem affected. Yes, I know they had a limited amount of time and couldn’t show everything. Within the world of the show, is there a plausible explanation? Maybe she’s so in control that it couldn’t affect her?
The substance can’t work on people whose legs are exposed to light. Chapel always wore a somewhat more lengthy, modest dress than did Uhura, and of course the men all wore pants.
Or maybe it’s not guaranteed contagious, and any given touch only has a chance of transmission. It might be a very high chance, but maybe Uhura just made her saving throw.
The intensity and speed of onset varied substantially among different people exposed to the substance, and the expression of symptoms was idiosyncratic. Sulu and Reilly were exposed at the same time, but Sulu showed symptoms some time before Reilly. Chapel’s reaction was somewhat delayed, and initially manifested as distraction, rather than overt action. Other crew members–lab personnel, the crewman with the paint–were apparently affected quickly and responded with hysterical laughter. Spock was affected immediately and had a serious breakdown.
Uhura did do something uncharacteristic at one point after exposure. She snapped at Kirk in response to an order. She caught herself and apologized (and so did he), but it was unusual for her, even in a stressful situation. All we really see from her, aside from that one snap, was her messing with her console and not being able to get anything to work. It’s possible that the stuff was affecting her by making her more frustrated and distracted. If so, barking back at Kirk and failing to take back the comm channels may have actually *been *her symptoms.
It could be that the infectious sweat came specifically from the hand glands. Uhura touching Sulu’s sweaty chest didn’t represent a vector for infection.
There was skin contact, at least between her hand and his torso. I don’t see any way she would not have been exposed to his sweat. She probably had more exposure than anyone but Reilly and Sulu, who wrestled Tormolen down and were exposed to his blood as well as his sweat.
I’m sticking with “She had symptoms, and they just happened to be less noticeable than the others.”