John sensed the evil presence of the Lamia but chose to believe it was an illness, and acted surprised when Merlin suggested it was sorcery. This may have been because he was afraid.
When Gaius sent Merlin to try and diagnose the problem, John had little faith in him. Sir Elyan had to reassure him he was to be trusted, saying he was handpicked by the King himself. John cared deeply for his village and loved his wife deeply. It is suggested that he is old and is traditional in a sense, saying the same thing at the same time every night: "There's a chill in the air tonight".
John's wife, Mary Howden, is an old friend of Guinevere. So, it is possible Guinevere is a friend of John Howden as well or that he at least knows her, considering John sent Mary to Camelot to ask for help instead of going himself. This could have been because he wanted to remain in the village with his people, because he couldn't move far for other reasons, or because he did not know Camelot very well (Lamia).