Hugo Chavez's US 'cancer plot' put to the numbers test

President Hugo Chavez
Image caption Venezuelan president Hugo Chavez had a cancerous tumour removed in 2011

Venezuelan President Hugo Chavez speculated last month that the US might have used a secret weapon to give Latin American leaders cancer, as the number of them with the disease was "difficult to explain using the laws of probabilities" - but is it?

"Would it be strange if they had developed the technology to induce cancer and nobody knew about it?" Mr Chavez asked in a televised speech to soldiers at an army base.

Treated for cancer himself last year, he was speaking the day after the Argentinian President Cristina Fernandez de Kirchner was diagnosed with the condition - or misdiagnosed, as it turned out.

Brazilian President Dilma Rousseff, 64, had treatment for lymphoma in 2009.

Her predecessor, Lula da Silva, 66, has been treated for throat cancer.

Paraguayan President Fernando Lugo, 60, was diagnosed with lymphoma in August 2010 but is now in remission after chemotherapy.

'Disease of the elderly'

This made five leaders out of a total of 24 Latin American countries, at the time of Mr Chavez's speech, although it became clear after an operation this month that President Fernandez was suffering from something else entirely.

President Chavez stressed that he was thinking aloud rather than making "rash accusations". The US State Department described his comments as "horrific and reprehensible".

But was he right that such a concentration of cancer is statistically improbable?

Cancer is a very common disease, points out Eduardo Cazap, an Argentinian doctor and the president of the Union for International Cancer Control, based in Geneva.

Over a whole lifetime, the risk of cancer is around one in three for women and one in two for men.

At any one time it affects about 1% of the world's population.

In the case of the Latin American leaders, Dr Cazap says, their risk is higher than that of the general population because they are all in their 50s and 60s. Cancer is a "disease of the elderly", he points out.

Another issue to consider is the fact that the Latin American leaders were not diagnosed with cancer in the same year, but over a three-year period.

So, if we make a specific adjustment for the age group of the population and then multiply by three, five out of 24 - roughly one in five - is not a very unexpected number, Dr Cazap says.

Cancer increase

It is worth noting that not all the leaders were diagnosed with cancer while in office - former Brazilian president Mr da Silva discovered he had throat cancer in the year after he stepped down.

Latin America currently has about 8 to 10% of the world's cancer cases, which is to be expected given its population of about 600 million - roughly 9% of the estimated world population of seven billion.

But the prevalence of cancer in the region is expected to increase "enormously" by 2020-30, Dr Cazap says.

"This compares to Europe, the US and Japan, where the cancer incidence will remain more or less stable in the next 20 years."

Dr Cazap says the main reason for this is that a number of Latin American countries are becoming more economically developed, which in turn is bringing rapid urbanisation and ageing populations.

As countries become wealthier, changes in lifestyle also occur, which lead to an increase in the number of people getting cancer.

High rates of smoking, obesity and a lack of physical exercise are also particular problems in the region.

So, given what we know about cancer in Latin America, can we be sure the US has not used a secret health weapon against Mr Chavez and the other leaders?

Apart from his misunderstanding of statistics - or the "law of probabilities", as he put it - another point to consider is that the different leaders have different types of cancer, and the biological mechanisms behind each are different.

Also, our bodies are in general pretty good at repairing any damage we do to them, which would complicate things for anyone trying to make a cancer-inducing weapon.

"Our body is extremely resistant to all the factors that could affect it. And when you need to produce cancer in an experimental manner you need to use huge amounts of drugs or huge amounts of toxins," says Dr Cazap.

Along with other health experts the BBC has contacted, he doesn't hesitate to conclude that Mr Chavez's "very imaginative version" of events is "difficult to apply to the reality".

More or Less is broadcast on BBC Radio 4 and the BBC World Service. You can listen again to the full programme via the BBC iPlayer or by downloading the More or Less podcast.

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