Aetna Inc., WellPoint Inc. and other big insurers are moving to ban payments for care resulting from serious errors. The companies are following the lead of Medicare, which starting in October 2008 will no longer pay the extra cost of treating bed sores, falls and six other preventable injuries and infections that occur while a patient is in a hospital. Some hospitals are concerned that the strategy could drive up medical costs as hospitals absorb or pass on the expense of introducing the safety and screening procedures needed to help avoid mistakes.