It seems that much of Season 5 revolved around Mac and girls - which to be honest was not as interesting as Tom being in love.
Anyway, he falls in love with Suzanne after her friend Dell Martin is rundown in a parking lot. After Suzanne becomes sick it is soon clear to her new biology tutor, Mac, that something isn't right - she is HIV positive. At first she denies that she got it from sex, but from sharing a needle, until the truth is revealed - Dell Martin's autopsy confirms that he had AIDS, and had know for over a year.
Touching storyline about Suzanne not wanting to hurt people and Mac in love again...well that does get annoying but it is done well in this episode.