You need to talk to your doctor. Your weight is a reflection of your health. Being 90 pounds isn't a bad thing, but being 90 pounds because you don't eat IS. Your doctor can tell you if you are a healthy weight and have a healthy lifestyle. If they recommend that you gain weight they will give you advice on how to do it properly.
PLEASE don't just make assumptions on your health without professional advice.
If severely underweight yes. Basically your body is starving to death. This effect body organs negatively which will eventually kill you. It takes time but it does happen. Increase callorie intake. It doesnt have to be nuts. Plenty of caloric food out there.