Both forms of the disease have long been thought to involve not only genetic but also environmental factors.

The decision to sell a drug by prescription, experts say, may involve factors that have nothing to do with science or patient safety.

But for Craig, the search has involved factors that many of his friends have not had to worry about.

The reasons involve economic, political, professional and human factors.

It can also involve factors such as lifetime costs and maintenance arrangements.

This sometimes involves such factors as whether or not the wheels have been removed.

Research with humans and animals also suggests that the condition may involve genetic factors.

But any emergency the size of this one involved public-health factors.

From the 1970s onwards, however, the moves began to accelerate and involved quite different factors.

The disease process is almost certainly complex and may involve several factors.