Take the full B vitamins, which usually include biotin. There’s something called Nail, Hair & Skin Connection that they sell in health food stores and vitamin shops.
It’s also is good for your nerves.
The only drawback is if you have hepatitis you’ll never know it. It’s a color thing.
Here’s another tip that is totally unconnected to the theme:
If you are taking vitamin D for germ and virus resistance, take minerals too or at least zinc. D gives you those annoying leg cramps in the morning, if you take a lot of it especially, and zinc somehow counteracts that.