Flat head in infants: how serious is it, doctor?

In 1992, the WHO recommended that babies should sleep on their backs to prevent cot death. Since then, cot deaths have plummeted, but as a collateral effect, flat-headed babies have appeared in large numbers. Half of all babies could be affected. What are the long-term risks for the child? How can we prevent a baby from developing plagiocephaly? And above all, how can it be treated?