It seemed to come from nowhere. And by the time we sat up and took notice, the Zika virus was rampaging through Brazil and on to more than 20 South and Central American countries, carried in the saliva of the Aedes mosquito.

Let loose in human populations that had no existing immunity, it has rapidly spread to millions.

For most people, the symptoms are negligible. But there appears to be one horrific consequence – for pregnant women.

Because with the arrival of Zika has come a 20-fold increase in the number of babies with small heads and underdeveloped brains, a condition known as microcephaly.

A few of those affected will die, and those that survive will have development problems.

The World Health Organisation has convened an emergency meeting, describing the spread of the disease as ‘explosive’, with some suggestions it could infect three to four million people in the Americas alone in the coming year.

Pregnant women have been advised against going to afflicted areas – which would include Rio de Janeiro; host to this year’s Olympic Games.

And because there is some evidence of sexual transmission, Public Health England has advised men to use condoms for at least a month after returning from an affected country if they believe their partner is at risk of becoming pregnant.

No wonder the authorities are increasingly alarmed.

And with the virus likely to spread to other areas where the mosquitoes thrive – for example the southern US and southern Europe including some Mediterranean resorts – the potential threat to British visitors is real.