Hesjedal's Garmin-Sharp team revealed Tuesday that the Victoria native developed an upper respiratory tract infection during the race, which he won in 2012. He has returned to his home in Girona, Spain since pulling out on Friday.
The team said Hesjedal entered the race in very good physical form, but was still vulnerable to illness. His effort in the Stage-8 time trial, combined with further efforts in rainy and cold conditions, stressed his immune system and made him susceptible to contracting the infection.
He completed the ninth and 10th stages, but his suffering continued and he pulled out before the 11th stage. At the time, he sat in 38th place overall, more than 32 minutes behind the leader.
The team said it was virtually impossible for him to continue competing, because the condition would have worsened.
"It appears that Ryder had been exposed to the virus affecting many in the peloton during the first week of racing," said team physician Phil Stawski in a news release.
"Secondary to his outstanding form and conditioning, the more typical symptoms did not manifest until later. However, the effect on his body and performance was already occurring.
"His effort during the (time trial) and in the cold and wet conditions continued to impact his body's ability to fight the infection, and he ultimately succumbed. The only option to prevent the worsening of his condition was to stop racing and allow him to recover."
Hesjedal is expected to be able to resume training in about a week. He plans to compete in Switzerland's Tour de Suisse, to be held June 8-16.
"It's hard to watch the Giro from home and not be there with my team. ... (But) The team's been great with helping me," said Hesjedal in the news release. "I appreciate all the support. I look forward to getting back on the road and racing again at Suisse, and will take it day by day from there."
Jonathan Vaughters, the Garmin-Sharp team's CEO, said the crew is looking forward to getting Hesjedal recovered, healthy and back on the road racing again.
"He was in great condition before the Giro, and while it's a shame that he got so sick, we're glad he is starting to come around," said Vaughters. "Our team staff will continue to work closely with him to get him healthy again. That's priority number one, but we'll all be glad to get him back to racing."