An epidemiologist points to a fifth sort of human malaria.

Malaria has plagued humans since the dawn of written history, and probably since long before that. These days, biologists understand tiny mechanistic details of the workings of one human malarial parasite, Plasmodium falciparum, but know surprisingly little about the others. As someone who studies how pandemics are born and die — and how they might one day be prevented — these holes in our knowledge seem striking to me.

Aside from P. falciparum — the cause of 'malignant' malaria — parasitologists acknowledge three other human malaria parasites, P. vivax, P. ovale and P. malariae, each of which probably jumped from another primate host to humans independently. With so many malaria parasites plaguing other vertebrate species, however, and only basic diagnostic instruments available in most parts of the world, science could be missing new types of human malaria that have the potential to seed pandemics.

In a recent paper, Janet Cox-Singh and her colleagues build on their earlier finding that humans can harbour a fifth malaria parasite, P. knowlesi, which was once thought to infect only Asian monkeys. The researchers detected P. knowlesi DNA in about one third of 1,014 malaria patients in Malaysia, showing that this parasite is common, deadly and almost always misidentified as P. malariae (J. Cox-Singh et al. Clin. Infect. Dis. 46, 165–171; 2008).

That an unknown animal pathogen can cause widespread human disease is reminiscent of some of the biggest scourges of the twentieth century: HIV and pandemic influenza. Reductionist, molecular approaches to tackling important plagues may be en vogue and a near necessity for grant funding, but I bet that an old-fashioned natural historian studying how infectious agents jump host species will be first to signal the coming of the next plague.

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