What do Lyme's disease in Long Island, a pig from New Jersey, and an amateur pianist have in common? All are subjects in three of 25 utterly fascinating tales of strange illnesses, rare diseases, poisons, and parasites - each tale a thriller of medical suspense by the incomparable Berton Roueche'. Roueche's true narratives have long been favorites for readers of ''The New Yorker.'' Now the best of these articles are collected here to astound new readers with intriguing tales of epidemics in Americas small towns, threats of contagion in our biggest cities, even bubonic plague in a peaceful urban park.