Why Asia should worry about Zika

It could be the plot of a dystopian thriller: the sudden outbreak of a disease that spreads almost invisibly, for which there is no cure and no vaccination, but which is linked to horrific deformity in babies and may cause some adults' immune defences to attack their own nervous systems.

But Zika is no fiction.

The spread of the disease across the Americas is being described as an 'explosive pandemic' and now Asia is on alert.

India has already started testing for the virus among its 1.3 billion population. And it will be no surprise if it is found, because India has a surprisingly long history of Zika infection.

Zika is a mosquito-borne virus which has recently been linked to shrunken brains in children and a rare auto-immune disease called Guillain-Barre syndrome.

Scientists who have been studying the outbreak in the Americas say a couple of million people have almost certainly been infected. The virus has been detected in 23 countries in the region and now threatens to spill into the US. So far Asia is reckoned to be Zika-free. Certainly all the samples analysed for the virus by the National Institute of Virology in Pune - India's centre of expertise on the spread of viral illnesses - have been negative.

But India has had Zika before.

When the Institute was first established in 1952, a group of scientists was commissioned to try to work out what diseases it should focus its efforts on.

They went out across India collecting blood samples to test for exposure to a list of 15 insect-borne diseases. Amazingly they included Zika on the list.

I say amazingly because Zika had only just got into the spotlight.

The virus was first isolated from monkeys living in the Zika jungle of Uganda in 1947.

It was only formally described as a distinct virus in 1952.

Yet the researchers discovered that 'significant numbers' of people had been exposed to the virus in India. A total of 33 of the 196 people tested for the new disease had immunity.

"It therefore seems certain", they concluded in a paper published in 1953 "that Zika virus attacks human beings in India". What is particularly extraordinary is that this conclusion was reached even before the first official case in a human being was registered in Nigeria in 1954.

It suggests, one expert on the spread of infectious diseases tells me, that Zika was already widespread even before that first live virus was isolated in a human.

The research team was not particularly concerned by the evidence of Zika infection.

Zika was regarded as a very mild illness causing just a slight rash and fever, and with no significant long-term complications.

That perception has changed dramatically over the last couple of months, as Zika has been linked with abnormalities in brain development in pregnancy and autoimmune illness.

Indeed, it is possible that Zika has remained endemic here but has simply not been identified because it is not routinely tested for.

And even if India does not already have Zika, the progress of two other diseases - dengue and chikungunya - demonstrates that it could spread.

All three viruses are carried by the same mosquito, the now infamous Aedes aegypti.

Just a few months ago I was out hunting Aedes aegypti in the narrow streets of Delhi's old town with a government team.

I was there because India suffered one of the worst outbreaks of dengue in its history last year. There were 25,000 confirmed cases, but the real figure was reckoned to be at least 100 times higher.

The spread of dengue across the world has been dramatic. The disease causes high fever and agonising joint pain. About one in a hundred victims die.

Fifty years ago it had only ever been recorded in a couple of countries. Now it is endemic in more than 100, putting more than half the world's population at risk.

There are reckoned to be 100 million cases of the disease a year, and perhaps as many as a million deaths.

And wherever there is dengue, one expert told the BBC, you are likely - in time - to get Zika too.

Professor Laura Rodrigues, a fellow of the Brazilian Academy of Sciences and the London School of Hygiene and Tropical Medicine, says she thinks there is a real chance Aedes aegypti will re-infect Asia with the virus.

Indeed, the findings of the Pan American Health Organisation (PAHO) are likely to apply to Asia as well as the Americas.

It anticipates that "Zika virus will continue to spread and will likely reach all countries and territories of the region where Aedes mosquitoes are found".

In the meantime the race is on to try to understand how the virus works on the human body and to try to develop a vaccine.

Even the most optimistic estimates suggest that will take a minimum of two years.

- Justin Rowlatt, BBC