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Malaria is one of the biggest killers worldwide. It's the fifth biggest killer in Africa (about 90 percent of malaria deaths occur in subsaharan Africa) and as of 2012, after a decade-long intervention program that slashed deaths by almost half in the region, it still killed 564, 000 people in subsaharan Africa alone and infected 207 million people around the world (Source: WHO Factsheet).
It's easy to think of diseases that have claimed similar numbers of lives, and which we have conquered, largely by vaccination. While the WHO calls malaria "entirely preventable and treatable," the preventions that are available take the form of mosquito nets and prophylactic chloroquinine or treatment with opther quinnine-like drugs, and the treatments are by African standards expensive.
There's a complication, too: the most dangerous type of malaria, falciparum malaria, is resistant to chloroquinine and often at least one other drug too. Quinnine still works, usually, but it's hard to get. And other treatments, like doxycycline, atovaquone and proguanil hydrocholoride, come with unwanted side effects.
The final nail in the coffin of a treatment-and-prevention program that could wipe out malaria is the animal reservoir. That's a group of nonhuman animals that carry the disease, so even if all the humans around you are malaria-free you can still catch it from an animal, and then infect your family, friends and neighbors. Some diseases, like flu, have animal reservoirs in the form of birds and pigs, so we get bird flu, swine flu and so on. We spend a lot of time with those animals, so it makes sense that we can share diseases back and forth. And it's thought that AIDS may have been an ape of monkey disease that crossed over to humans by hunting. But the relationship between humans and malaria's animal reservoir is different from that between us and our food animals — because they feed on us.
Experience suggests that what really protects children from diseases you can catch is herd immunity, which is why vaccination is so important. We've literally rid the earth of smallpox by vaccination, and came close to totally obliterating polio before the Taliban started telling Afghan mothers that the oral polio vaccine contained poison and would make their children infertile. (Now there's a massive polio outbreak in Afghanistan and other areas controlled by the Taliban.) We can't protect people in poor countries with expensive nets and long treatment programs. We need a vaccine.
So if we can vaccinate against so many other things, why isn't there a malaria vaccine already?
Vaccines work by teaching your immune system what a virus, bacterium or even a protovirus looks like so it can attack the invader before it breeds and causes damage. But polio, smallpox or even TB remain basically the same, mutating only very slowly. Malaria has a six-stage lifecycle and in every stage it looks completely different. Because it's not a bacterium; it's a plasmodium protozoa.