[userulluipeste]

"A farm atop a former factory in The Hague produces vegetables and fish in a self-sustaining loop: Fish waste fertilizes plants, which filter the water for the fish."

If people should know something about how diseases & parasites appear and develop is that the existence of exactly this kind of "self-sustaining loop" is the essential part. Even in the outside environment, any given organism bares the risk of taking part in development of a new parasitic creature's life-cycle, but that risk is greatly reduced by having an inconsistent pattern of interaction with other symbiotic creatures. Here the people involved are just asking for it (unless they go out their way to sterilize the substances circulating in that plant-fish loop).