Salary man Saki almost makes it home but passes out from fever just as college student Shinomiya passes by. When Saki comes to he realizes he's in a strange apartment and Shinomiya has been taking care of him for the past two days! Not wanting to be indebted, Saki tries to repay him as thanks but Shinomiya's having nothing of it. Luckily, Saki's pretty creative when coming up with a way to express his gratitude that has them both wanting more...
In the Kokoro series:
V.1 - Otokogokoro (A Man's Heart)
http://www.mangahere.co/manga/otokogokoro/
V.2 - Koibitogokoro (Heart of Lover)
http://www.mangahere.co/manga/koibitogokoro/
V.3 - Koigokoro (Heart of Love)
http://www.mangahere.co/manga/koigokoro_kanda_neko/
V.4 - Tokimekigokoro
http://www.mangahere.co/manga/tokimekigokoro/