In early 1999, Karen Cassidy, a dental hygienist and former high school athletics official, saw ads touting the new LYMErix vaccine for preventing Lyme disease, a bacterial infection passed to people by the bloodsucking bite of the deer tick. "Deer are right at my back door," says Cassidy, who hoped the vaccine would let her rake the yard of her suburban Philadelphia home without fear. But shortly after she completed two of a course of three inoculations in May 1999, Cassidy began experiencing burning pain in her back, numbness in her arms and aches and swelling in an ankle. "Just the thought of walking across the yard hurt me," says Cassidy.

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