Difficulty in opening the mouth is a symptom of..

Tetanus is caused by Clostridium tetani bacteria which produce a neurotoxin called tetanospasmin. This toxin causes severe muscle spasms and rigidity, classically beginning in the jaw muscles (lockjaw or trismus), making it difficult or impossible to open the mouth. The muscle rigidity then spreads to other parts of the body. Cholera causes watery diarrhea, plague causes swollen lymph nodes and fever, and diphtheria causes throat membrane formation.