2000 BCE - Here, eat this root.

1000 CE - That root is heathen. Here, say this prayer.

1850 CE - That prayer is superstition. Here, drink this potion.

1930 CE - That potion is snake oil. Here, swallow this pill.

1980 CE - That pill is ineffective. Here, take this antibiotic.

2000 CE - That antibiotic doesn't work anymore. Here, eat this root.