You could imagine a criminal seeding Ageratina altissima in pastures (possibly hybridized, as the plant is pre-adapted to mild temperate climates) or mixing it to feedstock.
The mixing can be done anywhere the feedstock is handled, and the plant is an infestant weed; at worst, it could be cultivate in a glasshouse. Seeding on the field, though, has the potential of making it all look accidental.
When ingested by cows, the toxin tremetol accumulates in the milk. The cow's behaviour changes, but not so much that a person unfamiliar with the symptoms would notice anything clearly amiss. Milk taste and texture remain the same and curdling does not occur.
Consumption of the milk results in intoxication with high probability of death (in malnourished individuals, death is a certainty):
The sickness has been called puking fever, sick stomach, the slows, and the trembles [...] In man, the symptoms are loss of appetite, listlessness, weakness, vague pains, muscle stiffness, vomiting, abdominal discomfort, severe constipation, bad breath, and finally coma. Recovery is slow and may never be complete, but more often an attack is fatal.
However, I do not think that "fearing your next meal" can be achieved if the poison is contained in the milk. Association with food is difficult in the first place, because consumption is not immediately followed by death, so people initially would not fear their meals because they wouldn't know the poison is there. Once the association is made, though, zeroing in on milk will be quite immediate, and at that point all that it's needed is to avoid milk and dairy products.
You would need some other reason why milk and dairy products are an obligatory food - either that or starve. In that case, yes, people would fear their meal since they wouldn't know whether or not it's poisonous.