Their bites transmit parasites and viruses that kill millions of people each year.

One of them, dengue virus, causes dengue fever.

It's often called breakbone fever

because the patients feel like the bones in their bodies are being broken.

If you get sick with dengue, you can have mild symptoms through to very extreme symptoms,

where you're going to have internal bleeding,

shock and can die.

So it’s a big spectrum

but if you speak to anybody that's had a bad dose of dengue,

they'll tell you it's one of the worst things that happened to them in their life.

Dengue is considered the world’s fastest spreading tropical disease.

Its cases have risen thirtyfold in the last 50 years.

Dengue is like everywhere in the tropics.

Potentially 40% of the world's population is at risk this year of getting dengue.

That's a big number.

We're talking about billions of people.

And with the world’s temperatures rising,

dengue can spread out of the tropics potentially reaching a further two billion people by 2080.

In Indonesia the total number of dengue cases is second in the world.

It has been proven very difficult to sort of

get rid of all the mosquitoes causing dengue.

In the laboratories chemicals are discovered for killing mosquitoes.

We’ve tried very hard but still, the mosquitoes are around us.

So it needs something else to combat dengue.

Eight years ago Scott O’Neill founded The World Mosquito Program,

a non-profit initiative running trials in 12 countries around the world,

one of them here in Indonesia.

Their sole aim is to eradicate dengue.

In particular one mosquito, Aedes aegypti,

it is responsible for transmitting between people a number of diseases.

Some of them obscure, some of them quite famous.

So diseases like yellow fever, dengue, chikungunya, and most recently, Zika.

And the reason that this particular mosquito, Aedes aegypti, is so good

at being able to cause these explosive epidemics

is because it's like the cockroach of the mosquito world.

Just like cockroaches, it likes to live with people.

It doesn't live out in the forest in harmony with nature or whatever,

it lives in people's houses, in concrete jungles, in cities,

and it bites people, maybe two or three times a day.

And so often whole families get sick with dengue,

because that one mosquito bit everybody in that house within a period of a couple of days.

As soon as people discovered that mosquitoes transmit disease,

the immediate response is let's kill mosquitoes.

The thing is all those diseases are still pretty well here.

And the reason is that mosquitoes are really tough to kill

and if you realize where they breed, you know just in a tiny bit of water,

you realize that there are millions and millions of places where these mosquitoes are breeding

and a thought of killing every single one of them

just seems impossible.

Most researchers have focused on genetically modifying mosquitoes,

or trying to eradicate the species entirely.

Instead, O’Neill’s team is infecting mosquitoes with bacteria - not to kill them,

but to inoculate them.

It’s a naturally occurring bacterium called Wolbachia.

That bacterium spreads into the mosquito population,

and once the mosquitoes have it, they're unable to transmit the virus between people.

You know, I’ve had this obsession for a long time of working on Wolbachia.

It occurs naturally in around 60 to 70% of all insect species all around the world so

wherever you live, if you were to go outside, grab some insects out of the nearest bush

you’d likely find, more often than not, that those insects

naturally have this bacteria called Wolbachia.

The mosquito though, that transmits all these viruses to people, doesn't have it.

When we were able to put the Wolbachia into the mosquito

and then we fed those mosquitoes in a laboratory virus,

we found that just Wolbachia by itself, without any fancy tricks,

would stop the transmission of the viruses.

Wolbachia was stopping, not just dengue, but yellow fever,

chikungunya, Zika, Mayaro, a whole range of viruses

and so it was a major discovery.

But introducing Wolbachia bacteria

to disease carrying mosquitoes like the Aedes aegypti isn’t easy.

In fact, the only way to do it is to inject freshly laid mosquito eggs with the bacterium.

This process takes time.

O’Neill’s team has spent more than five years building their colony of a few thousand mosquitoes.

But that was the hard part.

Now, nature takes over.

This bacterium transmits itself vertically from a mother to her offspring,

so it gets carried in the egg.

So what Wolbachia does is that if it's in a female it will be transmitted

and so that female can mate with either males that have Wolbachia, or males that don't

and she'll produce eggs, and those eggs will all have Wolbachia.

The trick though is the other way.

If the male has Wolbachia, but the female doesn't have Wolbachia,

then she will lay eggs and all her eggs will die.

The end result of that is that only females that have Wolbachia are able to reproduce.

And so Wolbachia then spreads into the insect population without having to be infectious,

without having to jump from one individual to another.

In Yogyakarta, whenever you go to the community and you ask,

”Do you know anyone that had dengue before?”

my guess is that everyone will answer "Yes."

Because it's so common, and it's still created a panic within the community.

People often, who live in transmission areas, live with a lot of fear about dengue.

Actually it's a family of viruses. It can be grouped into four groups.

And unimaginatively, they're called dengue 1, 2, 3, and 4.

When you get infected, say, with dengue 1, your body makes antibodies against dengue 1,

and so it's much harder for you to then acquire dengue 1 again, the second time.

But those antibodies don't protect you for dengue 2, 3, or 4.

But not only that, those antibodies will make it easier for dengue 2, 3, and 4

to get into your body and cause disease and actually create potentially more severe disease.

And so it's possible that you could get dengue four times.

And each time you get it, you could potentially become more sick and at greater risk of dying.

Yogyakarta is where the group is conducting a major study.

They’re collecting data on how successful Wolbachia is in stopping dengue transmissions.

The first batch of mosquitoes was released here in 2014.

We worked with the community far before we released the mosquitoes

to make sure that they understand that what we are targeting is the virus,

not just the mosquitoes.

This project would not have been possible without the community understanding.

Because the idea was like a controversy.

Let's think about it for a minute.

We're going to come in, a bunch of scientists

and we're going to release mosquitoes into the community.

And these mosquitoes are going to bite you.

And all you've heard for the last 50 years of your life is that you have to kill mosquitoes,

because they're dangerous.

You would imagine the community would be very cautious.

I'd be cautious or I’d wanna know a lot about what’s going on.

And I think community is like that, and we understand that, and we've really tried to address that.

And by doing that, we've had virtually no opposition.

Communities are a huge part of the project and the team spends time

explaining the science behind their intervention.

But they’re also involving them in the mosquito releases.

People take in the buckets with mosquito eggs and take care of them until the eggs hatch.

In this space mosquitoes are being grown that contain Wolbachia.

So that we can collect eggs off them and then those eggs will be distributed into the community.

In Yogyakarta we're using human volunteers for feeding.

Some of the researchers have been bitten probably more than a million times by mosquitoes with Wolbachia.

All for the sake of disease prevention.

The team keeps monitoring how many mosquitoes in the wild have Wolbachia,

and continues to release them until they reach a certain threshold.

Once this is done, the bacteria sustain themselves in the mosquito population

and the method doesn’t have to be repeated.

Current data shows around 70% reduction in dengue cases

in the areas where Wolbachia mosquitoes were released.

But we think it's a big underestimate, because if you spend a little bit of time in Yogyakarta,

you'll notice that everybody's on motorbikes traveling everywhere.

And so it's possible that they've gone out,

been bitten by a mosquito and then come back in and then get counted as in the intervention area.

And it looks like, you know, everything that we've seen in the laboratory, our mathematical modeling,

everything is coming together to suggest that we're gonna have a big impact on disease.

By the end of this year the team’s Wolbachia method will cover

about three million people around the world.

But they want to reach at least 100 million by 2023.

The ambition for our program is big, but the problem is huge.

Eliminating diseases is very challenging.

I think if you look at polio today, you can bring polio down to very low levels.

But to do the final elimination, so there is no more cases, very challenging.

Smallpox is an example,

there aren't too many others out there.

Spreading Wolbachia mosquitoes around half of the planet is a monumental task.

It would require a huge injection of funding and a coordinated effort of governments.

But the three billion people at risk of such dangerous viruses motivates the team to keep fighting.

There's an obsessive nature to the work that we do.

And I think that sits behind a lot of science, to have an idea and then to really hold onto it

and then work and work and work and work, until we’re successful.

And so my hope would be that we could eliminate dengue at some point.