Belly button odor can be embarrassing and can owe itself due to a variety of causes, ranging from the innocuous to the worrisome.
Bad hygiene, wherein link, dead skin or other debris getting caught can be the reason for the odor.
Then again it could be caused by obesity, wherein, there are folds of skin at the bellybutton and the area does not get enough fresh air.
The belly button is a dark and warm place and could also be moist, which is an ideal breeding ground for a number of fungal infections. And infection or an abscess could be because wherein the multiplication of bacteria can give off an offensive odor.
A yeast infection in the belly button could also be the cause of a bad smell. There could be itching in the area and vigorous scratching could well cause some bleeding in the area.
What however may be worrisome is if there is a discharge from the navel due to a urachal cyst or a previous surgery such as Patent urachus. This is a rare but potentially problematic condition, which would typically require surgery.