He thinks that war is a necessary evil.

My diet is rich in protein.

His constitution makes him prone to catching colds.

He has a susceptibility to depression, and tends to get down over little things.

He has cyclothymia, and his mood changes drastically.

The aerosol in this area is very dense.

The cytoplasm contains various substances necessary for cellular activity.

This wall is made of chalky material.

Protein is a nutrient necessary for muscle growth.

The white matter in the brain plays a role in transmitting signals of nerve cells.