General:
* Unrestrained
* Uncontrolled
* Widespread
* Prevalent
* Rife
* Out of control
* Unbridled
More specific:
* Epidemic (for disease)
* Pandemic (for disease)
* Outbreak (for disease)
* Bountiful (for growth)
* Thriving (for growth)
* Flourishing (for growth)
* Proliferating (for growth)
Negative connotations:
* Unruly
* Vicious
* Ruthless
* Savage
* Destructive
Positive connotations:
* Abundant
* Brisk
* Vigorous
* Robust
* Prosperous
The best synonym to use will depend on the specific meaning you want to convey.