28 Days Later2002

[Shortly after Selena has killed the newly infected Mark.]

Jim:
How did you know? I mean, how did you know he was infected?

Selena:
The blood.

Jim:
Yeah... but there was blood everywhere. It was on me, it was on you -

Selena:
Look, I didn't know he was infected, okay? He knew. I could see it in his face. Look, if someone gets infected you've got between ten and twenty seconds to kill them. It might be your brother or your sister or your oldest friend. It makes no difference. And just so you know where you stand - if it happens to you, I'll do it in a heartbeat... He was full of plans, Mark. Do you have plans, Jim? Do you want us to find a cure and save the world, or just fall in love? Plans are pointless. Staying alive's as good as it gets.