Welcome to Muppet Wiki!
The Birdy Pox is a disease Big Bird contracts in Episode 3434 of Sesame Street. First, the infectee gets green polka-dots on their feet, then the legs, then the back, the chest, the tummy, the face, the wings and finally the tail. The cure for it is rest for a day, which is odd as fatigue is not a symptom. It only affects birds of Big Bird's species and only if they are six years old.