ectogenous

from The American Heritage® Dictionary of the English Language, 5th Edition.

• adjective Exogenous.
• adjective Able to live and develop outside a host, as certain pathogenic microorganisms do.

from The Century Dictionary.

• Originating or developed outside of the host; externally parasitic: opposed to endogenous.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• adjective zoology, of a pathogenic organism Able to survive outside of its host