Stuck at very first, first Infected

[second.exodous]

Ah, you're not supposed to go there where he came.

Don't read if you wanna find the solution on your on...

Spoiler below!

You need to go back to the room where is the dead scientist and go through the locked door there...

So I'm susposed to wait until he leaves again? I can just sneak

Spoiler below!

to those double doors? I did that already tried that, I though, and it didn't work. I'll try it again tonight.