There is a plague. It has wiped out four-fifth’s of our people for the past three generations; perhaps more. This plague is spread by eating food fertilized by human feces. On a few occasions, our people have contracted it by men putting their boots–which have been walked in the feces-fertilized gardens–onto the table. Our people don’t know that our food is poisoned when it is fertilized by human excrement, but they have noticed those few occasions when men’s boots were on the table there was sometimes plague. That is because it is an abnormal occurrence. Conversely, they don’t notice the E.coli already in their food because it is reliably present.
If I were counseling these people, all my efforts would be to remove human feces from the fertilization process because I know how old wives’ tales get propagated, and I know how deeply people invest in their pet (and petty) superstitions. Therefore I would never mention shoes-on-the-table as a source of plague except in a known, specific, instance because–while five out of one hundred men put E.coli on the table with their boots–one hundred out of one hundred people are eating food poisoned in the field.