General:
* Ubiquitous
* Prevalent
* Common
* Extensive
* Generalized
* Disseminated
* Pervasive
* Dominant
* Universal
More specific:
* Rampant (used for something negative)
* Epidemic (for diseases)
* Pandemic (for diseases that are global)
* Diffuse (for something spread out)
The best synonym will depend on the specific context of your sentence.