Before trading even began Monday, there were three major deals with a unit of UnitedHealth's merging with Catamaran, Horizon Pharmaceuticals' purchase of Hyperion Therapeutics and Teva's takeover of Auspex.
All told, 2015 is on track to surpass the record set in 2014 for M&A. Through Friday, there were $414 billion worth of deals in 2015, $49 billion more than than at the same time last year, according to Dealogic.
"Merger Monday" is back. Here's how to trade it.