2000 B.C. - Here, eat this root.
1000 A.D. - That root is heathen, say this prayer.
1850 A.D. - That prayer is superstition, drink this potion.
1940 A.D. - That potion is snake oil, swallow this pill.
1985 A.D. - That pill is ineffective, take this antibiotic.
2003 A.D. - That antibiotic is artificial. Here, eat this root.