How does Fitz get captured while he is traveling with a caravan and how does he escape?

A group of King's Guard comes to the camp, and Fitz is recognized. He tries to lace the men's food with poison, but it does not seem to work. He is taken prisoner and put in shackles. After the guards moved away from the caravan, the men become sick from the poison, and within a few days they are all dead and Fitz is free.