NARRATOR: The Roman Empire.

At its height, home to one in five people on earth.

But add in a few hundred years of corrupt decadence and barbarian invasions, and oh, how the mighty do fall.

Attila the Hun was bad enough, but Rome had another enemy on the inside.

The city of ancient Rome was a lot like Hollywood.

Rich people parked in the hills, poor people lived down below until they died young.

Most likely cause of death?

Fevers caused by bad air.

Wrong reason, right words.

DNA pulled from fifth century graves near Rome matched the parasite that causes malaria.

And once scholars started looking, they realized the spread of malaria was a big reason the empire went to ruin.

After they connected the world, Romans brought home more than they bargained for, a disease that targets children and the poor, and the insects that help it spread.

And that's what brings us to Zika.

The nation's top health officials warning that the Zika virus appears to be a lot scarier than first thought.

Florida now declaring a health emergency.

At least nine cases.

FEMALE REPORTER (ON TELEVISION): Now more than 30 cases detected here in the US.

MALE REPORTER (ON TELEVISION): Already, 346 Zika cases are reported all over the country.

Everything we look at with this virus seems to be a bit scarier than we initially thought.

NARRATOR: When the Zika virus was declared a global public health emergency in February 2016, this was all that scientists had ever written about it.

But we've learned a lot more in the months since.

Here's the facts.

Zika gets its name from the forest in Uganda where it was first isolated.

Like its flavivirus relatives, it looks like an awesome soccer ball, but it's considerably less fun to play with.

Over the next 60 years, Zika popped up in the odd monkey or mosquito, but prior to a 2007 outbreak on the Pacific island of Yap, only 14 human cases had ever been reported.

Of course, that all changed in late 2015, when Brazil noticed thousands of babies born with abnormally small heads and brains.

There's now little doubt that Zika is directly to blame.

Unlike most viruses, not only can Zika cross the placenta from mother to child, but we now know it specifically targets developing brain cells.

Worse, Zika also kills radial glial cells, which normally act as a scaffold to shape the developing brain.

There's no treatment or vaccine, and symptoms are usually so mild that most people don't even know they have it.

The World Health Organization has resorted to telling women in Zika affected areas, don't get pregnant, which is a pretty good sign we don't have any good options.

Of course, the virus can't do all this on its own.

A few cases of sexual transmission aside, almost everyone who's caught Zika got it from one of these.

Only a tiny fraction of the more than 3,000 known mosquito species bite humans, but aedes aegypti is one of the worst.

It feeds during the day.

It likes being indoors.

It even sneaks up on people from behind.

Every mosquito breeds in standing water, but even a discarded candy wrapper can hold enough for aedes to lay its eggs.

Of course, that doesn't even matter, because aedes eggs could be dried out for months and still hatch.

Zika has continued to march north, and the US has seen a few hundred cases in travelers returning from Zika areas, including nearly 200 pregnant women.

So will Zika start spreading locally?

That depends.

We have the mosquitoes.

Aedes aegypti was accidentally imported decades ago and is present across the South.

And there's a good chance it's more widespread relative, aedes albopictus, could also carry the disease, just not quite as well.

If colder climate species like Culex are able to pick up the virus, there isn't much of North America or Europe that's out of reach.

To keep the virus from gaining a foothold, the secret is to keep from getting bit.

Mosquitoes can't pass it to each other, they need us in between.

There's still a lot of unanswered questions about Zika, but the biggest is, why now?

Well, the world's a very different place than it was 50 years ago.

There's more people, and they live closer together than ever before.

We migrate on massive scales, whether it's the Olympics, or refugees fleeing war, perfect ingredients for an outbreak.

It's likely Zika always had the potential to be dangerous, it just never got the chance.

And it's not just Zika.

We're seeing more weird outbreaks in general.

Dengue, Chikungunya, Schistosomiasis, a parasite that's carried by snails is now second only to malaria.

Yeah, bet you never thought you had to worry about snails.

And have we forgotten about Ebola already?

It's a scary new pattern.

Every year, another tropical disease pops up out of seemingly nowhere and we're caught off guard.

The biggest risk factor for these diseases isn't geography or genetics, it's poverty.

Zika thrives when there's lots of people around to bite.

But it helps if they don't have windows on their houses or their neighborhood is full of trash.

You might think these problems are far away, but most of the worlds neglected diseases, from Zika to that one with the snails, strike poor people in the 20 richest nations on earth.

In Houston's fifth ward, doctors are already finding diseases like Dengue, Leishmaniasis, and the ingredients are perfect for Zika to check in.

We should develop vaccines and drugs to fight Zika, but there will be another Zika, or Ebola, or weird snail parasite thing to take its place.

Maybe the best way to keep people healthy is to help them live better.

Because if the Romans have taught us anything, it's that everyone is vulnerable, no matter how high on the hill you build your house.

Stay curious.