If someone is not sick, is she healthy?

Not necessarily. 

There are three parts to health: physical health, mental and emotional health, and social health.

Sickness is part of the physical health.

She could have terrible relationships with other people, therefore not being healthy in the social health section of the triangle, or not get good grades, therefore not being healthy in the mental/emotional side of the triangle.

Small intestine because that is the organ which absorbs nutrients