TO EXTERMINATE a living species by accident is normally frowned on. To do so deliberately might thus seem an extraordinary sin. But if that species is Plasmodium falciparum, the sin may be excused. This parasitic organism causes the most deadly form of malaria. Together with four cousins, it is responsible for about 450,000 deaths a year, and the ruination of the lives of millions more people who survive the initial crisis of disease. Besides the direct suffering this causes, the lost human potential is enormous. The Gates Foundation, an American charity, reckons that eradicating malaria would bring the world $2 trillion of benefits by 2040.  
Malaria is one of the worst examples of the damage that transmissible diseases can wreak. But it is not alone. AIDS carries off fit, young adults by the millions and tuberculosis by the hundreds of thousands. Measles, whooping cough and diarrhoea together kill over 1m children a year. Parasitic worms and mosquito-borne viruses like dengue, though they take relatively few lives, debilitate many.
Campaigns have brought the toll down heroically. As recently as 2000, malaria killed around 850,000 people a year; likewise, since 2000 deaths from measles have fallen by 75%, to around 150,000. These successes are to be celebrated, but an even greater prize exists: to go beyond controlling infections and infestations and instead to eradicate some of them completely, by exterminating the pathogens and parasites that cause them. That has been accomplished a couple of times in the past, for smallpox (a human disease) and rinderpest (a cattle disease similar to measles). The end is reckoned to be close for polio (a virus that once killed and crippled millions) and dracunculiasis (a parasitic worm). But more must follow.
Swat teams
Some diseases are not suitable for eradication because the organisms that cause them hang around in the environment, or have other animal hosts. Others, such as tuberculosis, can infect people “silently”, without causing symptoms, so are invisible to doctors. But sometimes the culprit is a poverty of ambition. A list of five plausible targets—measles, mumps, rubella, filariasis and pork tapeworm—has hardly changed since the early 1990s, yet measles, mumps and rubella are all the subjects of intensive vaccination campaigns that could easily be converted into ones of eradication. And even though Swaziland is poised to become the first malaria-free country in sub-Saharan Africa (see article), only a few dare to make explicit the goal of ridding the planet of the disease. Hepatitis C should be made a target, too. It kills half a million a year, and affects rich and poor countries alike, yet new drugs against it are almost 100% effective and there are no silent carriers. Eradicating these seven diseases—the five, plus malaria and hepatitis C—would save a yearly total of 1.2m lives. It would transform countless more.
People argue that the cost of chasing down the last few cases of a disease is not worth it. If the mass-vaccination campaigns under way can lower the incidence of measles, mumps, rubella and so on in poor countries to something close to rich-world levels, the argument goes, that is surely good enough.
Well, it isn’t. A disease can bounce back. That is what malaria did in the 1960s, when political attention waned, and the parasites that cause it evolved resistance to drugs and the mosquitoes that spread it evolved resistance to insecticides.
Three big improvements underpin the argument for throwing eradication’s net more widely. The first is better communications. The technology for locating and monitoring cases of disease in poor countries, even when few and far between, has improved immeasurably in the past two decades with the spread of mobile phones and the internet, and the expansion of road networks.
The second is better medical technology. The reason filariasis is on the “possibles” list, for example, is the invention of ivermectin, a drug that kills the worm which causes it. The inventors of this drug won half of this year’s Nobel prize for medicine (see article). The other half was won by the woman who came up with an answer to drug resistance in malaria—a medicine called artemisinin, which has been crucial to the success of the recent push against the disease. (This time, alert to the risk of resistance, doctors have formulated it with other drugs to create combination therapies that natural selection finds hard to get around.)
Even better technology is in the pipeline. In the case of mosquito-borne illnesses such as malaria and dengue, genetic engineering promises ways of making the insects resistant to the pathogens that they pass on to people, of crashing the mosquito population, and even of attacking insects and pathogens with genetically modified fungi and bacteria. Genetic engineering also promises a wide range of new vaccines.
The third reason for seeking eradication is a change in political attitudes. The emergence of AIDS, in particular, made governments everywhere sit up and take notice. Last year’s west African outbreak of Ebola only reinforced the message. Political attention leads to better medical infrastructure. To deal with AIDS, new networks of clinics were created and staffed with trained personnel. These can serve as the backbone of the campaigns that would be the starting-point for many extermination programmes.
The Dalek doctrine
The list of candidates for such programmes should be extended as and when circumstances change. The biggest prize might be AIDS itself. Smallpox, the first target for eradication, was picked in part because the virus that caused it had only humans as hosts and could not survive independently for more than a few hours. It had, in other words, no hiding place. Both of these are true of HIV, the AIDS-causing virus. What is missing is the third ingredient for smallpox: a reliable vaccine.
Throughout history, humans and disease have waged a deadly and never-ending war. Today the casualties are chiefly the world’s poorest people. But victory against some of the worst killers is at last within grasp. Seize it.