Biting your nails is often frowned upon as a bad habit, but could it actually be making you healthier?
Your hands pick up all sorts of bugs, including HPV and salmonella. But the theory goes that by giving your body a little of what is bad for it, you're actually boosting its immunity.
This video explains.
See also: Foods that boost your immune system
See also: Seven signs of cancer that can be easy to miss