The cause is the most mysterious aspect of the disease. Commentators then and now put much blame on the sewage, generally poor sanitation, and contaminated water supplies of the time, which might have harboured the source of infection. The first confirmed outbreak was in August 1485 at the end of the Wars of the Roses, which has led to speculation that it may have been brought over from France by the French mercenaries whom Henry Tudor used to gain the English throne. However, an earlier outbreak may have affected the city of York in June 1485, before Tudor's army landed, although the record of that disease's symptoms is not adequate enough to be certain. Regardless, the "Croyland Chronicle" mentions that Thomas Stanley, 1st Earl of Derby used the sweating sickness as an excuse not to join with Richard III's army prior to Tudor's victory over Richard at the Battle of Bosworth.