How can I kill the infected enemies?

[Googolplex]

Hello.
There 's a way to kill the infected enemies?
In the game is no dynamit or sth. - and no kind of weapon.

There 's one infected with a weapon. This, who is coming when you get the saw.

But I kan 't kill them.... There 's a way to get his weapon or to get some dynamit?

Or can I get this only with cheating?