This is the first and only episode in the series in which Jonathan has the most screen time.
At the charity party the end of "Fever" Pete hands Clark a CD and tells him it's the Talon mix. If you look closely you'll see that it is actually the soundtrack to the series.
The Season 2 DVD contains a deleted scene in which Clark and Lex discuss Helen and her impending departure for Johns Hopkins. Clark encourages Lex to do something to keep her.
This is the first episode of the series to have no human or even sentient antagonist, just a disease. There had been two similar episodes previously, Nicodemus and Rush; in both those episodes though, the diseases had brainwashed their victims into becoming physical antagonists to fight. The only kind of physical antagonism here comes from members of the DCA, and even here, they're honest people, just trying to do their jobs, which involves saving lives. The only thing that makes them physical antagonists at all is the need to keep Clark's secret a secret.
Chloe's letter to Clark later resurfaces almost six years later in Instinct.