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Okay I just feel like being creative so I'm gonna throw some ideas out there.

The Germ Spore gun

How does it work? Think of the can of bug spray in any rats map. The gun will shoot a sticky, maybe even hardly noticable, substance onto a wall, anyone who comes into a specific distance of it will systematically be drained of life until the person dies or the affect wears off. Could be good for warding off flanking enemies. Any modifications are welcomed.

That's a pretty good idea, what if for instance it would be gas.
Wich affects your health if you walk by it but if you shoot on it or drop a HE/Flash grenade near it (or maybe a flare), it will ignite and you'll be on fire for some seconds, if of course, your near it. If not, it will just ignite and the spray is gone. Flame and blast armor could also protect you against it.

Some interesting modifications, but I'd suppose it would look more like a thin film on any surface, giving a tiny tiny indication that you might be running into the spore.