Abstract: Article 6 of the SPS Agreement presents a series of interlinked obligations for importing and exporting countries of diseased agricultural products. The Russia – Pigs dispute raises the question of when an importing country is justified in imposing a ban on products from exporting countries unaffected by the disease, on the basis of the fact that the country is part of the same customs union as another country inflicted with the disease. This Article contends that four distinct classes of cross-border and cross-product externalities ought to play in an important role when assessing this question in the future. It discusses the possible roles to be played by bilateral, sequential, pass-through, and supply chain externalities in propagating the transmission of agricultural disease across borders through trade.