oey wasn’t getting better from his wound. He was getting worse and worse. Major Martin and Sergeant Thunder told Albert and David what was wrong. What was it that Joey had?

From the text:

It’s tetanus, son,’ said Sergeant Thunder. ‘Lock-jaw they calls it. It’s written all over ’im. That wound of ’is must have festered afore we got ’im ’ere. And once an ’orse ’as tetanus there’s very little chance, very little indeed.’