Gill - specifically on handling wild birds - if they look manky - leave them alone. Most of what I pick up are road kills meaning they were (probably) in fairly good health up until the point that they, er, weren't. They will have (depending on species and individual) a number of mites, lice and other hangers-on which will remain with the host until they find another one. These are pretty specific critters requiring very specialised habitat, so don't worry about them - unless you happen to be dressing up in your best Fulmar-feather outfit
- most are very slow-moving and can be cracked with the blunt end of a pencil or brush as they hit the white backing paper (which I like to use as it helps me see the contours and tones better than 'neutral' backgrounds. Reverse side of old wallpaper is good. One type of parasite which lives on rooks is a broad winged flea-type of thing and they are truly gruesome. They appear fom out of the plumage, do a quick once-over round the corpse and disappear back into the feathers - uuuuurrgh! I don't do corvids in the house - they stay outer doors.
One final thing. When using dining table (as Mr Derry and I both do now) - make sure it's not you sitting where aforementioned specimen was lying previous to setting table
- or you could disinfect the table, if you're being pedantic!
regarding catching 'stuff' from dead birds - I suppose the main thing is - don't eat them or suck them. You should be ok then.