"Of course it's pitiful for the monkeys. Everyone feels the same — you see it and you don't want it. But the point is if you want something different then you have to make something different. It doesn't happen overnight."

Speaking to VICE News, Jeffrey Bajramovic, a scientist from the Biomedical Primate Research Centre (BPRC) in Holland, was refreshingly honest. What happens to the monkeys tested on inside the center — a not for profit laboratory which is the largest facility of its kind in Europe, housing around 1,500 primates — is horrible. Those sent for experimentation suffer pain and distress, sometimes severe, in studies that sometimes last for months, before ending their lives on an autopsy table.

But the tests they undertake contribute to the understanding of and development of vaccines and treatments for some of the world's most deadly and prevalent diseases. And in a grim paradox, as Bajramovic pointed out, the captive primates are also contributing to the development of alternative research methods that scientists can use so that ultimately, they don't have to test on animals at all.

It's a messy and emotional ethical dilemma that VICE News came face to face with when we gained rare access to the BPRC to see just what happens inside.

Watch the VICE News documentary: Experimenting on Animals: Inside the Monkey Lab here:

The BPRC's work, like that of all scientists doing medical testing on animals around the world, relies on a utilitarian argument: Subjecting a small number of highly sentient beings to a horrible life and death is necessary to reduce the suffering and death of huge numbers of others. Some "survival of the fittest" also comes into it — given that human beings are the most sentient of all, and highest up on the food chain — we experiment on those below us.

Over 70,000 non-human primates were used for research in the United States in 2010, according to the US Department of Agriculture. They include macaques, baboons, marmosets, and other monkeys, as well as some chimpanzees. Around 65,000 dogs, 21,000 cats, and 53,000 pigs were used, as well as hundreds of thousands of rabbits, guinea pigs, and hamsters.

Around the world, dogs, cats, and primates together account for less than 0.2 percent of research animals, according to scientific lobby group Understanding Animal Research, which also points out the number of animals used for medical testing is miniscule compared to the number of animals bred and slaughtered for meat consumption.

Mice, rats, fish, and birds account for the majority, with around 25 million used in US laboratories a year, while estimates for the number of animals used annually around the world vary from 60 to 115 million.

After speaking to people on either side of the debate, it was interesting how much they had in common — everyone agrees that testing on animals is deeply unpleasant, that it would be better if animals were not used, and that phasing out the use of animals in medical research is the ultimate goal.

What the animal rights advocates and the scientists disagree on is how much benefit medical animal testing has brought, how fast animals should be phased out, and how they should be treated in the meantime.

"People automatically assume that primates are a better model [for medical research] because they're so similar to us," Kathleen Conlee, vice president of the US-based NGO Humane Society, told VICE News. "But the differences are enormous when you come to their biology. When it comes to the immune system that's where the greatest differences are and that's often what we're studying."

Nearly 90 vaccines that had promising results in primates have failed in clinical trials in humans, said Conlee, adding that very few detailed scientific studies on the scientific value of primate use had ever been conducted.

One of the only major US studies, the Institute of Medicine and National Research Council's landmark 2011 report, Chimpanzees in Biomedical and Behavioral Research: Assessing the Necessity, concluded that "chimpanzees are not necessary for most biomedical research," and that there were very few cases where chimpanzee research offered valuable enough insights to "offset the moral costs" of the experiments. The Humane Society is now pushing for a similar study to be done on research involving other primates.

"Animals models are always very crude and aren't giving us the answers we need," said Conlee. "They are always going to have limitations, whereas non-animal models will only continue to improve. So why do we continue to put such a huge investment in these animal models, and very little in alternatives?"

However the scientific community is clear — yes, there are major biological differences between primates and humans and so, yes, using primates is a crude research method that often does not produce the required results, but at the moment it's by far the best we've got.

Huge gains have been made thanks to animal research, they say, highlighting deep brain stimulation used to treat Parkinson's disease and the anti-retroviral drugs that have turned HIV from a terminal illness to a chronic condition across much of the world.

"Primates are necessary for certain questions," Jan Langermans, BPRC's deputy director and head of its animal science department, told VICE News. "Primates are the only animals that show the same characteristics as humans when they have HIV [SHIV in primates] and with tuberculosis for example, the characteristics of the disease, the pathological consequences, everything you see correlates exactly with what you see in humans.

"To look for vaccines and drugs to treat these severe diseases, we have to understand them, find out what is going on in their early and late stages. We learn a huge amount from primates."

In the European Union there are very strict regulations on the use of primates for medical research, he added, with researchers having to demonstrate that the answers they are seeking can only be found out through using monkeys.

There are also strict welfare regulations, much stricter than in the US and other parts of the world, though not strict enough according to animal rights activists.

At the BPRC, all the monkeys — in a colony that comprises around 1,200 rhesus macaques, around 150 long-tailed macaques, and around 225 common marmosets — first live in family groups in relatively large outside enclosures. The cages have play equipment like climbing frames, ropes, tires, and mirrors, and from birth the monkeys are socialized to get used to humans.

Around 10 percent of the monkeys get sent for experimentation each year — young males taken away at four or five years old, the same age they would typically leave their family group in the wild.

They first go to the training unit, where they are taught to become compliant with researchers through a process known as "positive reinforcement."

Step by step, the monkeys are taught to get used to needles and ultimately accept injections by associating them with food treats. As they learn that needles mean grapes and raisins, they gradually begin to cooperate with researchers, sometimes to the point that they will offer up their arms or legs for an injection.

"Of course they don't like it, we have to be honest," said Langermans. "But they like the training and the interaction, and the majority are willing to cooperate. And less stressed animals lead to better results, because stress affects the immune system."

It's when the monkeys leave the training facility that things get really grim. They head to the experimentation facility, where they are infected with highly contagious diseases such as dengue fever and HIV, before undergoing tests and examinations for up to several months. As they suffer these illnesses and tests, they are kept in relatively small metal cages, of around 169 cubic feet (4.8 cubic meters), with one other monkey for company. The cages have bedding, a swing and puzzle feeders for stimulation, but they could be better, admitted Langermans.

The cages had to be small so that the monkeys can be closely monitored and treated when needed, he said, and to ensure the protection of animal caretakers who were dealing with highly contagious diseases. The cost and technical feasibility of constructing enormous buildings to very strict environmental protection rules were also significant factors.

"There is definitely room for improvement and it will improve in the future," Langermans said.

International legislation on the use of animals for medical research is built around a framework known as the Three Rs: reduction, refinement, and replacement.

Reduction means improving experiment design, data sharing, and technology in order to minimize the number of animals used per study. Refinement means improving welfare, improving training, and using better analgesics in order to minimize the suffering of animals that are used. Replacement refers to developing models that avoid or replace the use of animals altogether.

The BPRC has a unit dedicated to replacement, headed by Bajramovic.

His team works on creating cell cultures that can mimic the behavior of certain types of human cells, with an ultimate aim of replicating tissue structures, entire organs, and perhaps the entire body, one day in the very distant future.

There's an ethical prerogative to do so but also a compelling scientific one, he told VICE News, because of the shortcomings of animal models.

"If you start with an animal you immediately know whether [the thing you´re testing] is physiologically relevant or not, it's a real shortcut to developing a drug because you can see if the drug is doing what it's supposed to do," he said.

"On the other hand you often have difficulty understanding how and why it is doing it. With a cell culture you can really get to the bottom of it — it is better science because you have more control and more understanding on a molecular level."

The research was still at the very beginning, he said, and it was a very slow process because every step must be validated. "You have to show each time that what you have in the culture dish is the same as what you see in animals," and that, of course, requires more animal testing.

But along the way, the research helps provides answers as to why certain drugs fail in humans after working on animals, and "if we can just contribute a tiny bit to that understanding and making our science better, that will save animals," Bajramovic said.

Conlee, of the Humane Society, said it was disheartening how little money was invested by governments into developing alternatives compared to the huge amounts invested in primate research.

Conlee actually started her professional life inside a primate breeding and testing facility housing more than 4,000 monkeys. She led the behavioral team that looked after the animals' welfare, but ultimately left because of disagreements about how high standards should be and because she felt she could better help the monkeys from the outside.

Leaving the monkeys that were in her care behind was heartbreaking, she said, and she still has nightmares about them years later.

"I'm in my car trying to get to the facility and it breaks down, then I'm on my bike, and that breaks, until in the end I'm running trying to get to them," she said. "It's horrible but it keeps me driven to do the work I do."

Conlee fully accepts that right now, animals are still required for animal research. But far fewer than are currently used, she argues, and they could be kept in far better conditions and tested on in less painful ways. We are inflicting intense suffering on highly sentient creatures for studies that often provide questionable results, she believes.

Ultimately, it is a societal issue and a societal responsibility, said Bajramovic, which is why the BPRC believes it is important to be transparent about its work.

"If society were to say we want to stop and take the current situation as the status quo, then researchers would adapt to that, and stop," he said. "The consequence would be that all these people who have diseases which don't have a cure would stay that way. I hope that we will reach a point where animals are phased out completely but at the moment they are saving a lot of human lives."