1. Star Trek: Enterprise: A Night in Sick Bay
   https://www.youtube.com/watch?v=-bkSS3mfH-U
   https://www.imdb.com/title/tt0572172/reference

2. Star Trek: The Next Generation: Sub Rosa (1994) Season 7 |
   Episode 14

https://www.youtube.com/watch?v=RNESMHuFrrU
https://www.imdb.com/title/tt0708778/reference
Dr. Beverly Crusher crushes/romances on an alien ghost!

1. Star Trek: Deep Space 9: Profit and Lace (1998) Season 6 |
   Episode 23
   
   https://www.youtube.com/watch?v=EO0K3MEv8_c
   https://www.imdb.com/title/tt0708582/reference
   Episode including a gender transitioning ferengi.

2. Star Trek: VOYAGER, Threshold (1996) Season 2 |
   Episode 15
   An episode where humans travelling faster than warp 10 will immediately evolve into advanced slugs from the distant future!

https://www.youtube.com/watch?v=EYt-OI7LlTk
https://www.imdb.com/title/tt0708995/reference
An episode that has lead to real world academic trolling:
https://trekmovie.com/2018/02/15/paper-analyzing-voyagers-threshold-accepted-by-science-journal-more-star-trek-great-links/