An experimental drug from Biogen Idec Inc became the first Alzheimer's treatment to significantly slow cognitive decline and reduce what is believed to be brain-destroying plaque in patients with early and mild forms of the disease, according to a small study likely to reignite hopes of a treatment. Alzheimer's affects 15 million people worldwide, a number that is expected to grow to 75 million by 2030 without effective treatments, likely costing billions of dollars year.