The Rockland Board of Health will host its annual flu clinic at the public library Monday from 5 to 7 p.m. Town Health Agent Janice McCarthy said that flu season has already begun, but that it’s impossible to predict how long it will last. “We first start seeing the flu with the onset of the cold weather,” she said. “It’s not like hunting season where it has a start date and an end date. The flu shows up when it shows up and leaves when it wants to.” McCarthy said the town has a supply of about 80 vaccinations, and encourages those planning to attend the clinic to preregister by calling 781-871-1874, ext. 350.