Gwynne Dyer: Not a permanent global threat

IKA, the mosquito-borne virus spreading through the Americas that has been linked to thousands of babies born with underdeveloped brains (microcephaly), is just the latest new disease to spread panic around the world.

And wait! News just in that it can be sexually transmitted, too!

There is real cause for concern here. The virus is almost bound to spread to the rest of the world, except those parts with winters severe enough to kill off the two species of mosquito that bear it, Aedes aegypti and Aedes albopictus. And these mosquitoes are active during the day (unlike the Anopheles mosquitoes that spread the malaria parasite), so insecticide-treated bed nets don't offer much protection.

The World Health Organisation has declared a global public health emergency, and the media panic is building: first Aids, now this. We are too many, we travel too much, and new pandemics are nature's retaliation for our sins. Clearly the apocalypse is upon us.

Well, no, actually. New diseases have been devastating human populations for at least three thousand years, but no modern pandemic compares with the Antonine Plague of the 2nd century CE, the Justinian Plague of the 6th century, or the Black Death of the 14th century, each of which killed between a quarter and a half of the populations affected.

The worst pandemic of relatively modern times was the "Spanish Flu" outbreak of 1918-19, which killed between three and 5 per cent of the world's people. It was bad, but it hardly compares with the older plagues.

The slow-moving Aids epidemic has killed about 30 million people since the 1980s, or less than half of 1 per cent of the world's current population. Two million people died of Aids in the peak year of 2005, the number of deaths in 2015 was 1.2 million. New infections are also falling.

And Zika? So far as we know, it doesn't kill anybody - apart from some of the microcephalic babies, about a quarter of whom die because their brains are too small to control their bodily functions. The majority, who do survive, face intellectual disability and development delays. Four-fifths of the adults who are infected experience no symptoms, and the fever in those who do usually burns out in less than a week. Nor does the Zika virus remain in the body permanently: women who have been infected are advised to wait six months before becoming pregnant (although many will probably choose to wait longer).

Zika has been around for quite a while. It was first identified in monkeys in Uganda in 1947, and the first human case was detected in Nigeria in 1954. It gradually spread east across Asia, and started crossing the Pacific early in this century. But by the time it reached Brazil last year, it had suddenly mutated into a form that causes microcephaly in some of the babies of infected mothers.

Infectious diseases were probably not a problem for our pre-civilised distant ancestors, but since we began living in dense populations highly infectious diseases have been civilisation's constant companions. And for most of our history we had no way of controlling these diseases except quarantine.

In the past century, however, science has begun to get on top of the problem. Killer flu epidemics are still possible because the highly unstable influenza virus can mutate faster than we can create and mass-produce the appropriate vaccine, but smallpox has been eradicated and polio is on the brink of extinction: new polio cases have fallen 99 per cent in the past 25 years, and Africa is now entirely polio-free.

Even the ancient scourge of malaria is in retreat. Deaths from malaria have halved in the past 15 years, and the new "gene-drive" technology opens up the prospect of eventual eradication of the disease.

Now that Zika has become a problem researchers have started working on a vaccine, and in due course one will almost certainly become available. Another approach may be to target the species that propagate it by releasing genetically modified sterile mosquitoes to reduce the size of the insect population. It will take time, but we are not facing a permanent global threat. The glass is not half-empty. It is half-full, and still filling up.