One of the most common and considered probably theories is that the young Prince of Wales died of the sweating sickness. It was a very common illness during this time, one that many who have watched The Tudors will have heard about. Very little is still understood about the sweating sickness, such as why it happened or how to cure it. Some people survived, but many died and even the noblemen and women and royalty were not immune.
The theory for this comes from the fact that Catherine of Aragon was also sick for four months of their marriage together. If one had it, it is highly likely that they both succumbed to the illness. Catherine was just fortunate to recover from the ailment. She was not quite so fortunate to marry Henry VIII in 1509 and find her marriage annulled just over 20 years later.
Coincidently, Anne Boleyn was said to have almost died from the sweating sickness in 1528. There are a number of letters sent from Henry VIII bidding her to get well soon and showing hope that he would not lose her before he had the chance of marrying her. Like Catherine, she fought the sickness bug and made a full recovery. Also like Catherine, her marriage to Henry VIII was not successful and she faced a worse ending by being executed.
The sweating sickness was a form of the plague, which spread around Wales at the time Arthur was there. Henry VII didn’t realize that the plague was spreading at the time, and by the time they did find out it was too late to get the Prince and his new bride out of there and back home to safety.
There were many theories surrounding the sweating sickness. Looking into records of the illness, it seemed to favor the stronger and wealthier men, and was only in England between 1485 and 1551. Most would die within hours of contracting the disease, and those who lasted 24 hours would find that they would get better over the week. This could rule out the sweating sickness if the records are right, since Catherine was ill for four months. However, everyone is different and it could have been a different form of the plague that was similar to the sweating sickness.