Read this excerpt from "Mad Cow, Furious Farmer.” The disease was first detected in England in 1986, though some scientists think the very first cases started at least ten years before. Cows usually eat only grass and other plants, but farmers had been feeding them a meat and bone mixture that made the cows plumper. But somehow, at least one batch of the mixture had become contaminated with what was then an unknown killer: a prion.

By 1993, British farmers were reporting up to one thousand new cases of BSE a week. Governments all around the world reacted by increasing testing for the disease and not allowing any cows to be eaten if they were at risk of having the disease. As sheep can also get a prion disease called scrapie, they were tested as well. Farmers were angry that they were losing their livestock, and nobody knew how to protect cows and people from the disease.

Consumers were also in a panic when they learned about BSE, and for a very good reason: prions are infectious. If you eat a prion from a "mad cow” or sheep, you are at risk for developing a human version of BSE. The public felt betrayed that their governments had underreacted to the problem or covered it up.

Eventually, in 1997, governments began to ban farmers from feeding their livestock high-risk meat and bone mixtures. With that ban, the epidemic quickly peaked, and by 2010 had largely disappeared. Over the years, half a million cows and two hundred people had been killed by prions.

What is the main idea of this excerpt?