Stargate Atlantis Season 4 Episode 13

(McKay and Katie are trapped in the botany lab)
Katie: ...It's gonna be OK. Dr. Keller and Zelenka-
McKay: We don't know that, we have no idea if it's gonna be OK. For all we know, Zelenka and Keller could already be infected. We don't even know how much of the city is in quarantine! If this is serious - and we have no reason to believe otherwise - we could be looking at a city-wide lockdown!
Katie: You are thinking the worst case scenario.
Rodney: Hello, this is what I do! Someone needs to think ahead to the worst possible outcome to any situation, in order to properly defend against it!
Katie: Any situation? Even the good ones?
Rodney: Hey, it doesn't take much for a, a, good situation, to, to, turn ugly! One minute you're happily riding your bicycle back from acing your geo-mechanics mid-term, warm breeze in your face, next thing you know, you're flying through the air 'cos you just hit a pothole!
Katie: (Walks over and pats Rodney on the shoulder) Oh, Rodney! Did that happen to you?!
Rodney: The point is, one must always be on guard! You know, always looking out for the potholes. Now, you can call me a catastrophist, but it's a necessary burden I bear.
Katie: A mindset like that, and a person would live in a constant state of worry and fear.
Rodney: You forgot despair.