“…When Hugo Chávez recently took his third round of chemotherapy, the Venezuelan president opted to do so in Caracas rather than in Havana, where he had received previous treatments for his undisclosed type of cancer.

For some, this was a sign the fiery 57-year-old socialist might be getting better. For others, it was a political move designed to counter criticism that Mr Chávez was turning Venezuela into a satellite of communist Cuba.

Few topics are more controversial in Caracas. In return for billions of dollars of aid, about 50,000 Cubans already work in Venezuela, mostly as doctors but also in military intelligence. Together they make for a mysterious but pervasive presence that Héctor Pérez Marcano, a Venezuelan former guerrilla leader, has called Cuba’s “second invasion” of the country.

Mr Pérez, who participated in a first abortive Cuban invasion of Venezuela in 1967, says this second invasion is the fruit of a “political seduction” campaign by Fidel Castro to convince Mr Chávez to shore up Cuba’s teetering economy. “Fidel realised he could use Chávez as an instrument,” he says.

Even so, the result is a symbiotic relationship that has been key to shoring up both governments in Havana and Caracas.

Mr Chávez and Mr Castro – his mentor and hero – first spoke of the single nation “Venecuba” in 2005. Today, Venezuela sends some 115m barrels of subsidised oil each day to the energy-starved Caribbean island nation. In return, Havana sends professionals to work in Venezuela to bolster Mr Chávez’s “Bolivarian revolution”.

Carlos Romero, an expert on Cuba-Venezuela relations at the Centraniversity of Venezuela, calculates that accumulated aid from Caracas to Havana between 1999 and 2009 amounts to $19.4bn, about 70 per cent of Venezuela’s total foreign aid during that period. That includes $14.2bn in payment for professional services such as doctors and $3.4bn forgone by selling Cuba oil at less than market prices. Furthermore, Cuba owes Venezuelan state oil company PDVSA at least $5bn for late payment on the oil. Both governments declined to comment for this article.

It is not hard to find Cubans working in Venezuela’s barrios, or slums, bolstering social programmes that have helped sustain Mr Chávez’s popularity ahead of elections next year. Especially important has been barrio adentro, where physicians provide free primary healthcare to a third of the population, according to government statistics.

Halfway up the hillside to the east of Caracas in Petare, one of Latin America’s biggest slums, stands one of barrio adentro’s red-brick octagonal structures. Inside, a Cuban doctor attends a long queue of patients. He declined to be interviewed, explaining that he was forbidden to talk to journalists.

But one of the 1,800 Cubans estimated to have defected from Venezuela was more forthcoming from the safety of Miami. “We worked like mules, for a minimum wage … I had no life,” said the medic, who requested anonymity for fear of reprisals against her family. “With all the street violence, I never went outside, working and living in the same place. It was inhuman.”

About 30,000 Cubans work in Venezuela’s health sector, Mr Romero estimates, and thousands more work in areas such as sport, agriculture, telecommunications and industry.

However, the most controversial aspect of Cuba’s connection with Venezuela is its military influence, which is promoted by Mr Chávez.

“Yes, there is military co-operation, which perhaps worries the bourgeoisie,” said Mr Chávez last year. “Well, the bourgeoisie can rest easy. Everything Cuba does for us is to strengthen the fatherland.”

General Antonio Rivero, who stood down from the Venezuelan army last year in protest, disagrees. He says Cubans are closely involved in the organisation of security and defence systems.

“No Venezuelan soldier in his right mind can approve of such actions. They can be obeyed, because they are orders, but the surrender of our sovereignty cannot be accepted.”

With their careers on the line, most in the military may put up with the Cuba presence in silence, but it is likely to be a divisive topic in the elections next year.