I wouldn't expect too many infections from blunt trauma, they should require open wounds for the most part. I get them frequently if I don't have a soap industry set up. Once I have soap on hand they don't tend to be a problem. I still see them occasionally, but they don't generally get bad. Dwarven soap seems to be about as good as penicillin.
I wonder if we let a world go long enough if soap-resistant bacteria will develop?
Getting set up for making soap seems like a pain, but once you've done it a few times it's no problem, especially if you use workflow (part of dfhack). Just set up a wood furnace with a job to make ash and have workflow set to keep a few ash blocks on hand. Then a lye maker set to always have a few units of lye around. Then a soapmaker with a small nearby stockpile set to just have tallow, that takes from my main food stockpile. So the first few tallow created and put in the main food stockpile will be brought over near the soapmaker, and thus will probably not be used in roasts. Then set the soapmaker to make soap from tallow, with workflow set to keep 10 or so soap in stock. Then you never need to worry about it again.
Setting up a tallow-only stockpile is a major pain without dfhack, every creature in the game is listed with its fat and tallow (plus a couple with wax, and fluffy wamblers with pudge), and there are a lot of creatures. Oh, and several screens of forgotten beast fat and tallow as well. But with the ability to search in the stockpile settings screen, sorting the tallow from the fat, wax, and pudge in the Food/Fat list is easy. Making soap really, really benefits from dfhack.
Keith