2000 B.C.  Here, eat this root.

A.D. 1000  That root is heathen.  Here, say this prayer.

A.D. 1850  That prayer is superstition.  Here, drink this potion.

A.D. 1940  That potion is snake oil.  Here, swallow this pill.

A.D. 1985  That pill is ineffective.  Here, take this antibiotic.

A.D. 2000  That antibiotic is artificial.  Here, take this root.