The history of medicine.
2000 BCE: Here, eat this root; it will make you strong.
1080 BCE: Throw out that root and drink this potion. It is better for you.
250 AD: Get rid of that potion; it is bad for you. Take this herb instead.
1910: Get rid of this herb and take this potion; it is more effective.
1950: That potion is bad for you; here, take this drug.
2000: That drug is no longer effective; here, eat this root.
From Why can't I get better?: Solving the mystery of Lyme and chronic disease by Dr. Richard I Horowitz.
Pretty much sums it up eh?