2000 BC: Here, eat this root.
1000 AD: That root is heathen. Say this prayer.
1850 AD: That prayer is pure superstition. Here, drink this potion.
1940 AD: That potion is snake oil. Here, swallow this pill.
1985 AD: That pill is ineffective. Here, take this antibiotic.
2000 AD: That antibiotic doesn't work any more. Here, eat this root.
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