i work in a very large inner city hospital and we have patients like this come through quite often. most of the time, if they are alert and oriented and able to bathe themselves, i don't give them much choice but to bathe. i give them the option of setting them up to do it themselves or i tell them that i will be more than happy to assist them with getting clean. gown up, glove up, bring in two buckets like another PP said, and get scrubbin. sometimes if you put a little bit of shaving cream in with the soap it also helps with the smell.
alot of elderly patients like a PP said are not physically able to wash appropriately because they are too weak, have no access to family members, etc, and i take special care with making sure that they get a nice bath since their skin is usually extra fragile and they are more likely to get infection and breakdown quicker.
do try and remember that the person you are washing, regardless of why they are not clean (and some mentally ill patients will fight you tooth and nail to shower), still deserve dignity and respect. i've seen too many people laugh, gag, fan themselves... just stuff that shouldn't happen.
the smells are not pleasant, yes, but usually your hospital pharmacy carries peppermint oil, which is not abrasive to the respiratory compromised and you can dab it on some 4x4's and stick it in the room. the vicks trick under the nose works well too, i prefer burt's bees menthol lip balm in my nose... but stay away from anything scented--i can tell you myself as an asthmatic, i wheeze from certain perfumes and lotions, i can only imagine if someone has chronic bronchitis what it does to them.
this is also a good time to get your full head to toe assessment of the patient as well as their skin assessment done at the same time. you can learn alot about a patient from their bath.
good luck to you