This is Planet Money from NPR.

You hear this a lot.

The economy cannot go back to normal until there's a coronavirus vaccine.

It's the highest-stakes thing science is trying to solve right now, more than 90 different

vaccines are being developed by companies and universities.

But before we put all of our hopes on a worldwide coronavirus vaccine, let's look at the history.

Vaccines are difficult to pull off.

If you think about it, we've only made vaccines for so many things.

There's like 22 diseases that we have vaccines for in the US.

We have it for the measles mom's and rubella, polio, hepatitis A, hepatitis B.

But we don't have a vaccine for hepatitis C or HIV or malaria or West Nile or Zika or

a bunch of others.

Some viruses, there is no vaccine for them yet.

Sometimes there's just a treatment, a drug, something to treat a virus once we have it,

but not prevent us from getting it the way a vaccine does.

A vaccine super simply is just exposing yourself to a little bit of the bad thing so that

your body can create an immune response that protects you in the future.

But when did we first get the idea to do that?

Like how did it occur to us to inject ourselves with something that can kill us in order to

save us?

Hello and welcome to Planet Money.

I'm Sarah Gonzales.

...