Hi
My son bit his nails for about a year (from age 4.5 to age 5.5). We tried the product "no bite", that I bought from the pharmacy. They are either right on the shelf, or behind the pharmacy desk (ask them). I dont want to discourage you, but it did NOT work on him. We asked the doctor during his regular routine check up, and she explained that nail biting is usually a habit as a result of something else in his life -- like new school, boredom, nervousness, anxiety, etc.. And she said that if we could figure out the core issue and change it, the biting (at this age) will go away, normally. Well, it seemed like a difficult task, but I started thinking about when his habit started and how it had progressed. The habit started after he started a new childcare center in the afternoon. We decided to give him rewards if he stopped chewing his nails first for 3 days, then a week. You can definately know if your child is still biting his nails, because his nails would always be short:) Well, after the reward system, my son's nails started to look better. Then came the summer,and he didn't go to the child care center. By mid summer, he completely stopped the biting!
So to sum it up, I would say that a positive encourage system, combined with removing him from the environment that caused the biting habit did the trick. Good luck with you! In the mean time, don't worry too much about him being sick from nail biting. I agree with other moms --there are many other places that he could have picked up germs.