I'm not sure if this has been posted before (knowing this site probably) but whatever. So this technically isn't a glitch par say but you can abuse the hell out of it.

Steps:
1.Become infected in a online lobby(matchmaking). Optionally, you can legitly play until you get infected, more XP this way.
2.Find some high area, doesn't have to be too high to kill you just to damage you.
3.Jump off it, rinse and repeat. IW decided it was a good idea to give Infected players Pro Perks, even if they don't have the perk itself unlocked yet . What your doing is completing the challenge 'Feather Weight' which you unlock once you get Dead Silence Pro, until now of course. You should get roughly 25000-30000 XP while you complete the challenge .

After note:I just unlocked Dead Silence !

Notes from posters:

NikolaiLoverG935 wrote just saying, but not all the infected types give infected dead silence.

False. Some game types will not list Dead Silence because that gametype only gives what Dead Silence Pro does, negate fall damage. You'll technically have Dead Silence Pro in all infected gametypes.

MALOxRuNzThis wrote Plus you only get this 1 time, you should have made this topic called "Easy way to get Featherweight/and..." whatever challenges completed, since you can play infected and have deadsilence pro at the start...know what I'm sayin?