I keep thinking that the interlopers are doctoring some major food or beverage product with the chronic viruses that cause cancer, autoimmune disorders, dementia, etc. The most likely to me seems to be dairy products, like milk, as they are meant to be consumed within a short shelf-life, and are stored under refrigeration. Is this something that happens, and if so, how widespread is this practice now?

You have reasoned your way to the discovery of a key vector for inflicting mayhem on the world. It is an irony as well, being the perversion of the best substitute for mother's milk, the form of nutrition most associated with nurturing of the young, helping them to grow strong and healthy, as a continuation of the gift of life starting from their beginning as a nursing infant. The fact that it is a convenient conveyance of biowarfare agents would seem truly unthinkable, yet this is so. The dairy industry is routinely being commandeered through acts of sabotage to contaminate dairy products, milk primarily, with dangerous and deadly viruses. These are organisms that may not necessarily cause an acute illness. That would be too noticeable and raise suspicion and lead to an inquiry that might cause the discovery of this sinister plan. The viruses used are ones that are slow-growing and insidious in taking up residence within the body, and then slowly over time exerting a deleterious effect, often many years after ingestion so there will not be a noticeable cause and effect association. After all, if people drink milk every day, or frequently, it is taken for granted and no one would ever raise the question if one day someone develops rheumatoid arthritis, whether it was caused by their years of drinking milk. The very idea would be absurd and unbelievable on its face. But this is unfortunately what is taking place in your world, that the iconic representation of nutrition, especially for the young, is used as a weapon against them that can set them up for lifelong suffering and early death from chronic illness.