What is the main idea of the passage？

A. Scientific appraisal shows that multifactorial diseases are often due in part to environmental conditions.
B. In the past half century, scientists have been able to find only a few principles that apply to all diseases.
C. Each disease has single underlying mechanism that must be understood before the disease can be cured.
D. Many diseases that were once life-threatening can now be cured by antibiotics.