The correct option is B Contagious
'A disease that spreads by contact' is called Contagious, Option B. Option A, 'Infectious' means liable to be transmitted to people via environment. Option C, 'Contiguous' means sharing a common border. Option D, 'Contextual' means related or dependent on the context. Thus, options A, C and D are incorrect options and the correct word for the phrase is Option B, Contagious.