Can’t remember the name of a black and white movie

Good morning. I watched a black and white movie 25-years ago, and can’t remember the name of it. I thought it was “The summer complaint” or “The summer epidemic” or “The summer pledge” or “The summer” something… The residents of a small town during summer start to get sick. A visiting psychiatrist, who I believe is on vacation, helps out with the mysterious outbreak. Near the end of the movie, an older man (I believe he had a beard) is jailed on suspicion behind the sickness of the town. In his jail cell, he starts acting psychotic. The psychiatrist and another fellow stand in front of the jail cell watching the man’s behavior. The psychiatrist states that there’s a test he can perform on the man to verify if he is indeed psychotic or if he’s only pretending; to be let out on insanity. The psychiatrist is saying all of this to where the psychotic man in the jail cell can hear him. Then the psychiatrist puts a piece of paper in the hand of the psychotic man and lights it on fire, and the man holds onto it while it’s burning him. The psychiatrist then tells the man next to him that a psychotic man would’ve dropped the paper when it started burning him, and that the test was a trick. The man in the jail cell looks to go nuts after this is said. Can anybody please help me out with the name of this movie? I’d love to watch it again. Thanks

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