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Matt Ridley is the author of provocative books on evolution, genetics and society. His books have sold over a million copies, been translated into thirty languages, and have won several awards.

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The economic defeat of tuberculosis

TB was not cured so much as prevented by better housing conditions

My latest Mind and Matter column for the Wall Street Journal:

Peter Pringle's new book "Experiment Eleven" documents a shocking scandal in the history of medicine, when Albert Schatz, the discoverer of streptomycin, was deprived of the credit and the Nobel Prize by his ambitious boss, Selman Waksman. Streptomycin was and is a miraculous cure for tuberculosis.

Yet the near disappearance of tuberculosis from the Western world, where it was once the greatest killer of all, owes little to streptomycin. Mortality from TB had already fallen by 75% in most Western countries by 1950, when streptomycin became available, and the rate of fall was little different before and after. Scarlet fever, pneumonia and diphtheria all declined rapidly long before their cures were introduced.

Surprisingly, there is still disagreement about why so many infectious diseases retreated in America and Europe after 1800. A medical historian named Thomas McKeown made his name in the 1970s with a book arguing that not only could medicine take little credit until very late in the story (smallpox vaccination aside), but that even sanitation played a small part before about 1900. Instead, he claimed, better nutrition, caused by better farming practices, was the chief cause of better disease resistance.

McKeown's thesis produced a strong reaction from those who thought his aim was political-to rebalance modern health-care from cure to prevention-and that he had missed statistical quirks, like the stagnation of the trend toward less disease at a time when urban populations were booming. For example, the period 1830-1870 in Britain saw few improvements in mortality, despite rising living standards. Living in cities certainly encouraged waterborne diseases like typhoid and cholera that had rarely been a problem in even the poorest villages. Public sanitation eventually did much to change this.

The urban boom also gives a clue to TB's rise and fall. By all accounts, TB was not such a big killer in medieval times, when people stayed in their small villages. It was urban crowding that made the bacterium so successful and so virulent. In the latter part of the 19th century, many of the urban poor either shared beds, or took different shifts in the same bed. Though this had happened before in the countryside, in the cities people were on the move, coming into contact with new room-sharers. It was neither hygiene nor nutrition that defeated TB (though both helped) so much as better housing.

It also helped that after 1900 TB patients were often isolated in sanatoria and that after 1950 people could be cured. All these factors led to a fall in the "effective contact number" per infected individual, from over 20 in 1900 to under one today. The bacterium's opportunities to jump to a new victim grew fewer and fewer.

Does this mean that, in political terms, you can arguably score the defeat of tuberculosis for the private sector, through the improvement in housing caused by economic growth, just as you can score the defeat of, say, cholera for the public sector, through the building of sewers? Surely, it's less straightforward than that: Government drove a lot of slum clearance, while taxes generated by private wealth paid for sewers, vaccines and antibiotics.

With recent rapid economic growth in India, TB incidence has halved since 1990. In Africa, too, it's now falling slowly, albeit after peaking at high levels. Drug resistance may slow the decline but economic growth should mean it does not reverse.

There is another lesson: TB is unlikely to return in the West. Resistance to streptomycin is spreading, but while that might make some cases incurable, the economic and social conditions for TB to spread no longer exist. America saw its lowest- ever incidence (3.6 per 100,000) in 2010.

Here's the graph for Africa and South Asia: