If you were around in the ‘90s, and especially if you lived in the UK, you might remember all the panic about mad cow disease.

It’s a disorder that’s killed thousands of cattle since the ‘80s and ‘90s, and because a version of it can also infect humans, a lot of people were really freaked out. Today, thanks to safer farming, it probably isn’t anything you need to worry about.

And even though scientists still don’t know exactly how it works, we can say one thing for sure: Your steak is almost definitely probably not infected. [INTRO ♪] Mad cow disease is the common name for a condition called Bovine Spongiform Encephalopathy, or BSE. It’s a progressive neurological disease that affects cows. And over time, it causes severe brain and nervous system damage, which eventually leads to trouble standing and walking, and changes in mood, like increased aggression and nervousness.

The “spongiform” part of the name just means spongy, because the infection creates a bunch of holes in the cow’s brain where its cells should be. The first two cases of BSE were identified in the UK in 1986, but it takes a really long time for symptoms to show up, so scientists think the first infections probably date back to the ‘70s. During the outbreak’s peak in 1993, almost a thousand new cattle were infected each week, but that number has gone down dramatically since then.

BSE is caused by a bizarre, self-replicating protein called a prion. Other pathogens, like bacteria and viruses, use DNA to make copies of themselves, but a prion is just a deformed version of a normal protein that’s found in cell membranes. Sometimes those proteins can go rogue and get bent out of shape, but right now, we don’t totally understand how or why it happens.

And when proteins become prions, they can bind to other proteins like them and make them bend in the same way, and then those messed-up prions corrupt even more proteins, and so on. Clumps of them collect and spread in the brain and nervous system, eventually causing brain damage. But that takes a while, so symptoms usually don’t show up until years later.

All of that is terrible, but how BSE got started is almost as horrifying. It probably happened because cattle were being fed ground-up meat and bones from sheep and other cows. Ugh!

There’s another prion disease in sheep called scrapie, and it’s possible that scrapie prions may have jumped to cows through their food and caused this whole mess. But it’s also possible BSE just showed up when a random protein folded the wrong way. No matter how it started, the cattle feed only made things worse— because after cows died of BSE, they were ground up and fed to healthy cows, so the disease kept going.

Unfortunately, a version of the disease can infect people, too. The human version of BSE is called variant Creutzfeldt-Jakob disease, or vCJD, and it’s also caused by bent prions. We don’t know for sure that it comes from eating infected cattle, but since they’re both prion diseases and both outbreaks happened around the same time, most scientists think that’s the case.

Like with mad cow disease, the symptoms of vCJD can take years to show up, but once they do, things move pretty quickly. Brain degeneration happens in just a few months, with symptoms like trembling, dementia, trouble walking, and eventually a coma. Since no cure exists yet, patients usually die within a year.

Worldwide, there have been about 230 cases of vCJD, and about 180 of those were in the UK. The rest were mainly in Europe, and there have been only four cases in the United States, but they were all picked up overseas. The rate at which people have been getting infected has gone way down since the ‘90s, but scientists will keep studying it because there are other kinds of Creutzfeldt-Jakob disease not caused by cows.

Before mad cow disease was a thing, we knew about CJD as a rare condition that could appear if a random protein went bad, usually through an inherited mutation, or through medical procedures like transplants. Today, we keep the risk of variant CJD low by giving cows safer food and by making sure no nervous system tissue gets into our beef. And even though the risk is small, you also can’t donate blood in the U.S. if you spent too much time in high-risk countries or got a blood transfusion in Europe, because there’s a chance prions could be spread through blood, too. Mainly, it’s those new farming practices that have really helped get the disease under control, and in 2016, there were no new reports of BSE in cows in the UK for the first time since the outbreak started. Which is both exciting and a relief.

Also, because mad cow disease is transmitted through nervous system tissue, there’s no evidence you can get it from milk or the meat used to make things like hamburgers and steak. So your roast beef is not out to get you. Which is always good to know.

Thanks for watching this episode of SciShow, and special thanks to Patreon patron Scott Sorrell for asking us about mad cow disease! If you’d like to submit your questions or just support the show, you can do so at patreon.com/scishow.