2000 B.C. -Here, eat this root.
1000 A.D. -That root is heathen. Here, say this prayer.
1850 A.D. -That prayer is superstition. Here, drink this potion.
1940 A.D. -That potion is snake oil. Here, swallow this pill.
1985 A.D. -That pill is ineffective. Here, take this antibiotic.
2000 A.D. -That antibiotic is artificial. Here, eat this root.