During his time as a veteranarian, he discovered an alien tapeworm in a cow near Cardiff. It infected him, then found it induced ravenous hunger and weight loss in its host. After experimentation he found out how to control and kill it. He used the eggs as part of a diet plan for his patients.
However, his control over their life-cycle using herbs was not absolute. He also did not know until after he had prescribed his weight loss pill to a number of patients that the tapeworm could cause psychosis if it grew too large. Torchwood Three were able to give him the drug he had developed to kill the tapeworms. Scotus' parasite, though, was so integrated with its host that he died as well. (PROSE: Slow Decay)