Allergan (AGN) was the best performer on the S&P 500 Monday after Teva Pharmaceuticals (TEVA) agreed to buy its generic-drug business for more than $40 billion. That's the latest in a surge of M&A in the health care industry, combined deals of which have already reached $180 billion this year. Mylan (MYL) tumbled after Teva withdrew its $40 billion offer for the company in favor of the Allergan deal. Mylan's board had proved resistant to Teva's hostile takeover attempts. It was a new week but no fresh start for Wall Street as stocks extended losses into a fifth straight day, their longest losing streak in six months. Another selloff in Chinese equities pressured global markets. Keurig (GMCR) was higher despite a federal appeals court resurrecting a lawsuit which accused the coffee company of misleading its shareholders.