I'm a bit confused about when and how concessions are offered. Some of what I've read seems to indicate that you can offer a concession when you take a consequence, but the book says you have to concede before the hit that takes you out, and if you don't want to take a consequence you are taken out when your stress track is bypassed.

I think the root of my confusion is partly whether you can offer to concede when it is not your turn. If anyone can point to someplace that has a more detailed description or example of a concession, or if you could talk a bit about how have seen it run or run it I'd be very grateful.

The Poison upgrade for Claws might fit your needs.

We did look at this, but my player and I agree the bacterial bite isn't something that keeps attacking and dealing stress on its own until the target is taken out, it's more that the Bite requires more or better medical attention to recover from.

Speaking of aspects, why not make it a power that allows for a maneuver?  So instead of damage it places an aspect.

So similar to the Venomous Claws upgrade, you apply the specific "Septic Wound" aspect rather than Poisoned, which can only be removed by proper medical attention, and until then can be tagged/invoked normally and also for a tracking by scent bonus.

Instead of the consequences one higher thing...why not require a Scholarship (first aid) roll equal to the value of the attack before the consequence will begin to heal.

I'm not sure how this would work with recovery powers though...

I like this idea rather than mine. Might be a book keeping problem though, depending on the situation. Maybe use the base values found in the book +2, kinda like tagging the consequence to oppose the treatment. So a Mild Septic Wound would be Great difficulty rather than just Fair.

Ok, where I'm at now
Bacterial Bite[-1 or -2]
Your bite is septic. Make a Fists Maneuver, and if successful, the target gains the "Septic Wound" Aspect. While this aspect endures, any rolls to track this target by scent gain a +1 to the roll. In addition, any consequences taken by the target while they have the "Septic Wound" aspect requires more skilled medical attention: treat the difficulty of the Scholarship roll to treat such consequences as +2 higher than normal.

I think this is slightly better and more focused now. Any further suggestions, such as the refresh cost? Thanks again for the input everyone!

Looks good to me. Thanks for the input, all.