Black plague - kill infected??

[windbrand]

I've watched a dozen of those "how to kill infected" videos. I did exactly as those videos showed, repeatedly hit them with objects, but it simply doesn't work? It seems in later stages the infected have some kind of immunity cooldown after being hit once, because if you try to hit them again too quickly they don't get stunned at all?? I'm at the "shelter", there are around 3 infected here, any tips on killing these?
I know you can sneak past them, so don't bother saying "you're supposed to sneak past them".

[Robby]

You have to hit them a lot of times. The higher the difficulty of the game, the more hits you may need. I did that kind of thing by accident. By tossing a box at an infected for 6-7 minutes.

[Yuhaney]

You need to hit them for a very long time before they die. Few hits won't do anything.
I bet even 5 sticks of dynamite won't kill them immediately.