“I must go down to the sea again,” wrote the poet John Masefield, “to the lonely sea and the sky.” The title of that poem is Sea-Fever, and we remember it at this season each year, when a similar affliction strikes Island residents and visitors. Forget Lyme: Derby fever is the disease in which the bite of a fish is not the cause, but the cure. Its characteristic mark is a derby button with the entrant’s number, and another indicator of a bad case is a vehicle festooned with rod holders.