Optimal voice function is dependent upon multiple factors, not just a pair of vocal folds. This story about opera singer Charity Tillemann-Dick demonstrates that for all voice users, not just professional singers: http://goo.gl/cwjBv. Charity suffered from a rare lung hypertension disease and had to undergo a double lung transplant, and yet she still continues to sing!
Sufficient respiratory drive is required to set the vocal folds into vibration, which is accomplished by a healthy set of lungs as well as the physical ability to drive the air through the vocal folds. In other words, you need enough air moving with the right amount of power thought your body for your voice to make sound. Good pulmonary health and overall physical well-being support the demands of ongoing daily voice.
Medical illness and medications can interfere with normal voice function. Taking care of your voice involves daily vocal hygiene such as adequate water intake, vocal warm-up and cool-down exercises, but attention to your overall physical and mental well-being.
Sometimes your voice only sounds as good as you feel.
