The plasmodium’s most effective defence is its complicated lifecycle, during which the organism morphs into completely different forms while inside our body, each presenting different antigens to our befuddled immune system.

A bite from an infected mosquito introduces around 100 sporozoites into the victim’s blood, which circulate only for up to 30 minutes before migrating to the liver, where they hide in its cells. Here, they multiply over the next 7-10 days, morphing into merozoites, which burst out of the liver cells in protective sacs, evading the immune system. Once in the bloodstream, the sacs disintegrate and the merozoites invade the red blood cells, rapidly and exponentially multiplying and destroying the blood cells.

This destructive "blood phase" causes the periodic fevers associated with malaria. Some of the merozoites then morph into sexual forms of the parasite, male and female gametes. These will then be ingested by a biting mosquito and, within the insect’s abdomen, morph into ookinetes, which burrow into the gut wall and form oocysts. Inside the oocyst, new sporozoites form, ready to be spread by the mosquito to a new victim.

The sporozoites are only in the blood briefly and there are too few of them for the immune system to reliably recognise and mount an attack, and by the time the merozoites are circulating, the parasite is so damaging and so fast-changing that the immune system again struggles to gain control. As soon as our bodies manage a response against one type of antigen on the merozoites, the exponential explosion of new forms outdoes this effort.

So to combat each new infection, a human host must mount a new and “strain”-specific immune response to each antigenically distinct parasite in that mosquito bite, as well as the new antigens that arise during the course of the infection. Because of this, a single malarial infection can be prolonged over many months to years. Only when a sufficiently wide spectrum of parasite strains has been experienced is any immunity achieved.

Every individual exposed to endemic malaria faces a long and dangerous battle to achieve even partial immunity to malaria, and the most vulnerable are the youngest. Very young children appear to have a poor capacity to acquire effective anti-malarial immunity of any sort, but anyone who hasn’t previously experienced the endemic strain of infection also faces great threat.

Protection from invaders

Historically, this protected Africans from colonial expansion. Europeans arriving on the continent died in such great numbers that the coast of Sierra Leone was known as the White Man's Grave. Malarial mortality rates exceeding 50% per year of contact were the norm on the West African coasts. And, despite the great cost of achieving immunity, it is readily lost – several months without reinfection are enough to leave an individual vulnerable to the full impact of malaria.

As a result, places that experience less frequent epidemics of the disease among the indigenous population can record higher death rates than those where it is endemic. Everyone I speak to in Ghana is aware of the protective benefits that come with the regular infections that they all experience. They are made more bearable through drug treatments.

The first effective treatment came from Peru. Malaria was introduced to the Americas probably in the transport of West African slaves, and indigenous people were observed treating the fevers with the bark of the Cinchona tree. In the mid-19th Century, quinine, the active antimalarial component in the bark, entered widespread use among Europeans in West Africa, rapidly reducing mortality rates by more than 75%.

In the 20th Century, a new drug chloroquine proved effective at treating and also preventing malaria, and this, in addition to the use of insecticides, such as DDT, was revolutionary. The insides and outsides of human spaces were sprayed, and malaria deaths plummeted. As prosperity increased in Europe and North America, fewer people were exposed to infection, and entire countries were declared free of the disease. In the 1950s, the dream of global eradication of malaria seemed just a decade or two away.