If a disorder occurs only in the presence of factor x but only when factor y or factor z are present, we might suspect that factor x is a cause of the disorder.

necessary but not sufficient

Explanation:

If a disorder occurs only in the presence of Factor X but only when Factor Y or Factor Z are present, we can say that Factor X being present is necessary for the disorder to occur, but if Factor X on its own cannot trigger the disorder unless other factors are present, then we can also assume Factor X is not sufficient to start the disorder. This "necessary but not sufficient" situation can be best understood in practical terms: if we have a car, this car by itself can't take us anywhere unless we also have other factors present, such as gasoline in the tank and the keys to start the car. Thus, in order to take a car ride, having a car is necessary but not sufficient, as we also need some other factors.

In medicine, this idea was developed by researcher Ken Rothman, who developed the sufficient-component cause model to explain how medical disorders and diseases spread. For example, following this model, it can be pointed out that being exposed to a person carrying the HIV virus is a necessary but not sufficient cause to become infected with the virus, as there are other factors that need to be present to contract AIDS, such as engaging in unprotected sex, sharing hypodermic needles, and absence of antirretroviral drugs.