Which statement correctly describes the relationship between a sufficient cause and a necessary cause?

• A. A sufficient cause can produce disease, while a necessary cause is required in all cases.
• B. A sufficient cause is always present in every case.
• C. A necessary cause can produce disease alone.
• D. A necessary cause is the same as a component-cause.

Answer: (A)

A sufficient cause is a set of factors that, taken together, will produce disease in a given context. There can be more than one such set for the same disease, so a disease can occur through multiple distinct sufficient pathways. A necessary cause, on the other hand, is a factor that must be present for the disease to occur in any case; without it, disease cannot happen, though its presence alone might not be enough to cause disease.

The statement is best because it captures both ideas: a sufficient cause can directly produce disease when its components are present, and a necessary cause is required in all cases for the disease to occur. This means there could be several different sufficient pathways, but there is at least one factor that must be present in every case.

Why the other ideas don’t fit as well: a sufficient cause isn’t required in every case—there can be alternative sufficient pathways; a necessary cause does not imply the disease can occur on its own without other factors, since it’s often not sufficient by itself; and a necessary cause is not the same thing as a component-cause (a component-cause is a piece of a sufficient cause, whereas a necessary cause is a condition that must be present in all cases).