Sentence 1: Bill smelled something horrible in the room. 
 Sentence 3: That's when his sister realized the stench was coming from his mouth 
 Sentence 4:  When he went to the doctor they found he had a mouth infection 
 Sentence 5:  Once he started on antibiotics the smell cleared right up 
 Option 1: But nobody else could smell it. 
 Option 2: It was a dead rat under the stove.