"external agent" is a correct and usable expression in written English.
You can use it when referring to an individual or entity that is outside of a particular system or organization and is responsible for bringing about changes in it. For example: "The company was able to successfully transition to a more efficient system thanks to the help of an external agent."
"The entire model depended on the ability of an external agent — the United States — to absorb trade deficits," he added.
He said only about 10% of total outstanding social welfare debts were referred to an external agent.
Here, heat is the external agent used to cause the model material degradation.
FK Whatever their status and location, all people are apprehensive of change, especially if that change is from an external agent.
They also exhibit efficient nuclease activity towards supercoiled pUC19 DNA both in the absence and presence of external agent (H2O2).
Autotomy, also called Self-amputation, the ability of certain animals to release part of the body that has been grasped by an external agent.
Whereas, in a miasmal view of the world, the distribution of disease followed the contours of morality — your nose just knew it — infection by an external agent smacked of moral randomness.
Some vehemently insist that an external agent (environmental hazards, a vaccine) caused their child to become autistic and seek out various treatments (some highly experimental) to "heal" their child.
The angle an iceberg 949 m in width (or length) and variable thickness H must be rotated by an external agent to overcome an energy barrier and capsize due to instability (blue curve).
And from here it's an easy step to the assumption that, given one external agent and one organism, there will also be one cure". Simple explanations, even if they are inadequate, appeal to common sense - and also make headlines.
One fact is so obvious that it's unfortunate I have to mention it: no external agent, such as a vaccine, can generate a consistent pattern of duplication and deletions in an affected individual's cells.
