太多有毒食物。

Too much acidic food.

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伯格博士说：“传染源现在可能已经不存在，也就是说，有毒食物可能不是被吃掉了就是被扔掉了。”

“It is possible that the source of the infection has now been exhausted, that is to say, that the food has either been eaten or thrown away,” Dr. Burger said.

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一些乳草属植物含有一种高浓度的化学物质，叫做强心苷(也叫卡烯内酯)，这种东西使得蝴蝶对那些食肉动物来说成了有毒食物。

Some milkweeds have a higher concentration of chemicals called cardenolides, which make the butterflies toxic to predators.

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However, their biological relatives in Mexico would presumably have that same risk, but it would never be manifested if they're not exposed to what some have called a toxic food and physical activity environment.

而且,他们远在墨西哥的血缘亲属,可能也面临同样风险,但他们面临着所谓的有毒食物和活动环境,这个说法就不会得到证实

耶鲁公开课 - 关于食物的心理学、生物学和政治学课程节选