These die-hard fans have been supporting the Chicago Cubs since birth.

Chicago's Advocate Children's Hospital began dressing newborns in "Born to fly the W" onesies on Oct. 7, and have since challenged Cleveland's UH Rainbow Babies & Children's Hospital to a social media battle to see who can get the most likes and shares on their baby photos.

The two hospitals have until the end of the World Series to rack up Facebook engagement from fans. The losing hospital's president will have to wear the opposing team's apparel and post a photo on social media.

Both hospitals are accepting photo submissions of kids -- including newborns and children up to 18 years old -- sporting their Chicago Cubs and Cleveland Indians gear via Facebook.