Immune System

• Underweight people often have weaker immune systems, which can make them more susceptible to infections and complications from surgery. A weaker immune system can also cause your body to take longer to recover from an illness, according to WebMD.
  
  

Hormones

• Being underweight can affect hormone balance in a woman's body, causing irregular periods and increasing the chances for infertility.
  
  

Bone Health

• Low body weight contributes to reduction of bone density, which can lead to an increased risk of osteoporosis.
  
  

Other Problems

• The composition of hair, teeth, and skin is also affected by a low body weight. Severely underweight people, such as those suffering from anorexia, can experience dry skin and hair; dry, brittle bones and teeth; and hair loss.
  
  

Gaining Weight

• Discuss weight gain options with your doctor, especially to identify any eating disorders. Healthy weight gain can be achieved in part by eating plenty of nutritious meals and snacks, incorporating "good" fats into the diet, and having balanced meals.