Runner's Companion has a number of rules for Infected and other nasties that include the Vulnerability (Substance) quality. However it doesn't seem to show up in SR4/RC/Aug so....anyone know what it does?

Running Wild, p219. This question comes up a lot. On the one hand, don't feel bad you couldn't find it; on the other hand, feel bad you didn't search. (Just kidding, welcome to Dumpshock!)
http://forums.dumpshock.com/index.php?showtopic=33909
http://forums.dumpshock.com/index.php?showtopic=33352

Vulnerability (Mundane)
Creatures with a vulnerability to a specific substance take
additional damage from weapons made of the substance. Increase
the Damage Value of all attacks with the substance by 2. Weapons
made of a substance to which the creature is vulnerable bypass any
Immunities that it may have. Damage taken from a substance to which
the creature is vulnerable cannot be healed by Regeneration or magic.

page 219 Running wild

Mea culpa on search-noobery.

Thanks for the quick response and welcomes. Ya'll seem like real chummers.

When it comes to stuff like this, yes.

Otherwise, well, Rule One: Don't feed the animals.

Rule two: We're all animals.

For a certain definition of animal.

Mammals ARE Animals, after all...

We do like ditties...

You and me baby ain't nuthin' but mammals...

...so let's do it like a druid in the general channel.