It has laid waste to armies, brought empires to their knees and killed half the humans who walked this planet. photojournalist John Stanmeyer travelled the world to document the terrible power of mosquito-borne malaria

AS you read this, I am in Liberia, West Africa. It is November: the end of the rainy season in a country that is among the wettest on Earth, its hot, humid climate providing the ideal breeding ground for the mosquito. The author Graham Greene came here in the 1930s, before penning his now famous travel book, Journey Without Maps, which documented the appalling health conditions that prevailed in a place where venereal disease and malaria were rife.

The last time I was in Liberia, the country itself was embarking on a journey. It was a journey without limits in terms of the barbarism experienced by its citizens. That was in 2003: a time when, wracked by anarchy and civil war, Liberia was a hellhole.

In the tumbledown capital, Monrovia, with its mouldly damp-blackened buildings, it was commonplace to see doped-up teenage gunmen, unnervingly attired in women's wigs, nightdresses and other voodoo regalia they believed powerful "juju" or magic, capable of protecting them from the bullets of rival killers. Rarely have I been in a spookier place.

Liberia is quieter now, its people trying desperately to rebuild their lives and heal the scars of a collective horror any sane person would find difficult to comprehend.

Today, those freakishly dolled-up marauders no longer roam the city streets and villages. But another indiscriminate killer still does. A killer that is, without question, the female of its species.

Attracted by the odour and warmth of living bodies, and armed with a piercing proboscis, it favours the quiet time between dusk and dawn when its victims are asleep.

Just like Liberia's homicidal militiamen, the female anopheles mosquito makes no distinction in its bloodletting. Women, children, the elderly and infirm have all succumbed to its scourge. In the thousands of years that this insect has been on the planet, drilling into the blood-filled capillaries and squirting countless malaria parasites known as plasmodia into its victims, it has ravaged humankind more than all of history's armies combined.

Malaria, like the demon in The Rolling Stones song Sympathy For The Devil, has indeed been around for many a "long long year". The disease has always been a global health threat. Only the plague and soon, perhaps, HIV, have had a greater influence on our demographic history. Some scientists believe that of every two people who have ever lived, one has died of malaria.

It is probable that the mosquito and its parasite were here long before humankind, making our dinosaur predecessors among its earliest victims. Signs of the disease have been found in Egyptian mummies and the skeleton of a child buried in a Roman cemetery. DNA evidence suggests that it may have contributed to the downfall of the Roman Empire. More than 2000 years ago, the Greek historian Herodotus noted that in swampy areas of Egypt, some people would sleep in tall structures that mosquitoes could not reach, or under special material that the insects failed to penetrate.

It is probable that Malaria stopped in their tracks the armies of Genghis Khan and Attila the Hun, and killed as many soldiers in the Far East during the second world war as did bullets and bombs.

As for the present and future, the statistics - current and projected - are terrifying.

Today, malaria, which takes its name from the Italian for "bad air", affects more people than ever before, stalking more than 100 nations and threatening half of the world's population. Of the one million people who will die this year following the symptoms of fever, shivering, headaches, vomiting and joint pains, most will be in such places as Liberia and other sub-Saharan African countries.

The vast majority are aged under five years old, and from impoverished families. Malaria is the plague of the poor. In many parts of Africa it is not unusual to find families with an income of less than £100 a year spending a quarter of this hard-earned money on malaria treatment.

It was among the poor and dispossessed that, some 25 years ago, I first encountered the disease. Shortly afterwards, I contracted it myself.

I had come, not to Africa, but to Central America, as a freelance correspondent on assignment, spending months living with guerrilla fighters and refugees in the steamy hot, mountainous forests of Nicaragua and El Salvador. At some point during that time, the anopheles mosquito did its work. Like most victims, I was probably bitten at night, the insect injecting me with the one-celled plasmodia parasite when I was sleeping deeply after yet another long, exhausting day in the sweltering bush.

Scientists say that as many as 50,000 plasmodia could swim in a pool the size of a full stop. But it takes only one to enter the bloodstream and make its way to the host's liver, where it multiplies and infects the red-blood cells, to trigger malaria.

Four species of plasmodia normally infect humans, and by far the most dangerous is Plasmodium falciparum. It is also the most prevalent, accounting for more than half the cases of malaria worldwide, and over 90% of deaths.

Rarely has nature created a more lethal pairing than that of the mosquito and falciparum. While a virus like measles consists of just a few genes, malaria is made up of 5000, with an ability to multiply rapidly and resist drugs used to treat the infected.

My own malaria would prove very resistant to efforts to treat it. But before then came the crushingly debilitating symptoms.

The first flush of fever hit one afternoon, as I interviewed some Salvadoran refugees - many of whom were infected with malaria - in a place called Colomoncagua on the Honduran border. By evening, it had taken a colossal grip.

"You're 106," announced one of the North American Jesuit priests who worked helping the refugees, after pulling a thermometer from my mouth.

"I'm pretty sure its malaria. We'll have to get you out of here to a hospital in the capital," he said, as I struggled to focus on the blurred face in front of me.

I spent the next few days laid out in the back of a pick-up truck en-route to the Honduran capital Tegucigalpa, in the throes of delirium. In the worst cases of malarial fever, temperatures can climb so high that the sufferer boils to death. Through the interminable shaking, burning and cold, I remember little except for the reassuring voice of an Italian photographer colleague, Massimo Martino, promising me endlessly: "We will be there soon David, very soon."

It would be another week after making Tegucigalpa and receiving initial treatment that I finally arrived in the UK. There, at London's famous St Pancras hospital for tropical diseases, my diagnosis was confirmed. So, too, was amoebic dysentery. According to the doctors, this, combined with the anaemia caused by the malaria, meant I was lucky to have made it home.

Malaria's great menace lies in its capacity to strike people already exposed to other debilitating diseases or conditions, such as dysentery, tuberculosis, bilharzia or malnutrition. The pairing of the mosquito and falciparum are a devastating combination which, when allied with hunger and poverty, make for a dual assault sometimes described by humanitarian workers as "the perfect storm".

Understanding the full force of this storm, and the role malaria plays in it, involves a journey into some of the world's poorest communities.

Places like the town of Kasongo in South Kivu province of the Democratic Republic of Congo. It was there, in the town's dilapidated hospital, that I encountered tiny, three-year-old Kabemba Ndarabo, spreadeagled on a table, malnourished, riddled with worms and infected with malaria.

The hospital was equipped with wooden storage cupboards half-eaten by termites; radiological equipment layered with dust; recycled surgical gloves; and stinking, faeces-filled toilet cubicles.

All of this was evidence of a public health system that had broken down, in an area where malarial exposure is among the highest anywhere. In such places, it's not unusual to be bitten by an infected mosquito 700 times a year (around twice each night).

Little Kabemba Ndarabo's chances of survival were slim, despite his mother's determined 30-mile-long hike to bring him to the only hospital. Malaria's burden is harsh in terms of death, but it goes far beyond that. When one child is dying of the disease, a mother or father is often forced to ignore the others. If the parents themselves are afflicted, their families probably won't eat, making them, too, vulnerable to illness.

Kabemba's mother had marched those long miles leaving behind two other children. Since her husband was killed in the civil war that gripped Congo, she had been the only breadwinner. How, I asked, would the other children eat in her absence?

"Our neighbours in the village will feed them the best they can. They don't have enough for themselves but they will give something," she replied, highlighting once again the knock-on effects malaria has on communities living a subsistence existence.

Malaria, like measles or polio, is no longer perceived as a threat across the developed world. Occasionally, we hear news of a traveller who has fallen victim to the disease. But for the most part, it is considered an exotic thing of the past, dismissed easily with another gin and tonic: a mixer which contains the quinine known to be useful in combating malaria.

But in the slums of places such as Dhaka in Bangladesh, where washing means scooping up stagnant water from ponds full of raw sewage surrounded by swarms of mosquitoes, malaria is an ever-present grim reaper. Here, some residents have devised a system of "hanging latrines" - precarious bamboo platforms raised a few feet above the water, and screened with rags. In the rainy season, the sluggish water rises above the tops of the stilts supporting some huts, flooding the floors and tiny alleyways with dead vermin, human faeces and other refuse. Malarial-carrying mosquitoes congregate in dense clouds. And one in 10 children here will die before they are five years old. During a visit last year, I heard one man compare living in Dhaka's slums to a state of "semi-death".

According to researchers interviewed by National Geographic, one of the most unfortunate things about malaria is that prosperous nations have rid themselves of it, thus ensuring little outcry about its prevalence elsewhere. "In the meantime, several distinctly unprosperous regions have reached the brink of total malarial collapse, virtually ruled by swarms of buzzing, flying syringes," observes the magazine's writer Michael Finkel.

The discovery that mosquitoes carry the parasite that was then passed to humans, was made in 1876, by British scientist Patrick Manson. Some four years later, French doctor, Alphonse Laveran, identified the culprit as the plasmodium parasite, while in 1897, another British scientist, Ronald Ross, found the malaria parasite in the anopheles mosquito.

These discoveries, and the launch of anti-malaria campaigns following the second world war, led to considerable success in combating the disease. Perhaps most notable was the discovery of the insecticidal effects of the compound known as DDT (dichloro-diphenyl-trichloroethane) by the Swiss chemist Paul Muller, who in 1948 was awarded the Nobel prize for his work. By the 1960s, however, DDT's success was discredited, as evidence mounted of its effect on wildlife and that the ever stubborn mosquito had become resistant to it.

After more than 100,000 cases of malaria were recorded among US troops during the Vietnam war, the US government invested heavily in the development of anti-malarial drugs. But in the decades that followed, the threat to US troops passed and the United Nations-backed global antimalarial campaign faltered in the face of the tenacity of the mosquito species. Western interest seemed to tail off.

Today, however, the race to find a vaccine has become a holy grail for big drug companies such as GlaxoSmithKline and charities such as the Bill and Melinda Gates Foundation. Gates, the world's richest man, seems to be on a personal crusade to rid the world of malaria. "It just blows my mind how little money has been spent on malaria research," he told New Yorker writer Michael Specter in 2005. "What has prevented the rich world from attempting this? I just keep asking myself, Do we really not care because it doesn't affect us?'"

At last, though, the world appears to be waking up to the threat posed by malaria, and the possibility of a "miracle" vaccine may be a little closer. A few weeks ago, scientists and campaigners welcomed the early results of a malarial vaccine prototype trial in African infants. Although researchers insist that caution is needed in interpreting the data from the study involving 214 babies in Mozambique, the vaccine has so far appeared to be safe, and highly protective. After three months, infants who had received it were 65% less likely to contract malaria than those without.

The quest for the vaccine represents a partnership between several African nations, the pharmaceutical industry and the PATH Malaria Vaccine Initiative (MVI). "These results essentially provide another green light indicating that we can move towards a large phase-three trial with this vaccine," says Christine Loucq, director of MVI. That trial is expected to begin next year in 10 sites across sub-Saharan Africa and, if successful, the vaccine will be licensed in 2011.

Such a success would mark a phenomenal step forward in the fight against malaria and the "audacious goal" Gates defined as "a day when no human being has malaria, and no mosquito on Earth is carrying it".

Last week, before leaving for Liberia, I visited a pharmacy to stock up on mosquito repellent and anti-malarial pills. Tonight after dusk in Monrovia, I will use that repellent, swallow my Malarone and make sure my mosquito net is tight shut against those nocturnal killers.

Elsewhere in Africa and around the world, countless millions of others, too poor to eat let alone pay for this protection, will be bitten. And every 30 seconds, another child will be killed by malaria.