Bad bug bites. Sunburn. Dirty pools. Heatstroke. The hazy, lazy days of summer, eagerly anticipated by New Englanders, can harbor a dark side, too. Illnesses from disease-bearing insects are on the rise. Public swimming pools may be teeming with bacteria, a recent study concluded. And long-awaited new labels for sunscreen products don’t tell consumers all they need to know to sufficiently protect themselves, say critics. Here’s a primer on common health concerns as the first official day of summer approaches.