They die young here.

Those who make it past infancy face a living hell. There is no other way to describe it. In the Swahili language Korogocho means "crowded shoulder to shoulder". It's a word that gives its name to this slum and the claustrophobic conditions that exist in many others such as Mathare, Kibera and Mukuru Kwa Njenga that scar the Kenyan capital, Nairobi. Almost all tourists, even the most liberal, come to Africa to see animals and wilderness. Very few come to see Africans. Fewer still ever set eyes on Kenya's children of the slums, for to enter their dark urban world is to venture into places of unimaginable filth, disease and violence.

In a makeshift clinic just off a tumbledown street of corrugated tin-and-wood shanties in the notorious Grogan A district of Korogocho, you will find the newborn of the slums. The rainy season downpours are leaking through the clinic roof as we step inside to be immediately confronted by a long queue of mothers with infants waiting to see one of only a handful of community health workers who provide for the estimated 200,000 slum dwellers crammed together in Korogocho, a space less than a single square mile.

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"The conditions here are worse than those you will find in many of East Africa's refugee camps," Adan Mohamud, a local community health worker, tells me. "Truly life here is extremely hard," he says, shaking his head despondently.

"Recently, along with a group of Danish medical students, I visited the home of one of these mothers," Adan says, pointing to the queue that has formed outside the clinic. "Inside there was only a dirt floor. No bed, no mattress, and she had six kids, none of whom had had breakfast that morning."

Adan went on to explain that to make matters even worse, the mother had been diagnosed as HIV-positive and had neither the 40 Kenyan shillings (30p) needed for the bus fare to the only hospital where she could get medication, or anyone to look after the children in her absence.

"The medical students I was with were in a state of shock after what they saw of the conditions in which this family lived. Some were in tears," he said, before adding that this was "commonplace in Korogocho".

Inside the clinic, which is supported and funded by the humanitarian agency Concern Worldwide, we watched as health workers, part of an outreach therapeutic-feeding programme (OTP), weighed and measured tiny infants suffering from malnourishment.

Last year, Concern Worldwide admitted 2839 children with severe malnutrition into its OTP, which covered six out of nine districts in Nairobi. Of those, 64 died.

In the first three months of 2011 alone, 1021 severely malnourished children from eight Nairobi districts were admitted and moderate malnutrition was identified in another 800.

I was all too familiar with the scenes at the Korogocho clinic that day, having witnessed them many times before in remote rural parts of Africa, or regions hit by drought, famine and war. But we were just a few miles from Nairobi's chic downtown bars and cybercafes, where people sip cappuccinos, and eat pizzas and steak.

Aid workers have called it the "hidden hunger", an urban food crisis that especially affects children in large cities such as Nairobi but rarely gets the media attention of food crises in other parts of rural Africa.

The World Health Organisation defines the nutritional situation as "critical" when 15% of children are malnourished, triggering a humanitarian response. With malnourishment in urban settings often overlooked, critics argue that this definition needs to be revised. For example, following one recent survey, children living in Nairobi's slums received no assistance as the malnutrition rate was an "acceptable" 3.5%.

Yet, in slums such as Mathare, home to tens of thousands of people crammed into an area two miles long by one mile wide, a malnutrition rate of 3.5% translates into 2961 children.

In slums like Korogocho and Mathare, it is not unusual for children to eat just one small meal a day. Sometimes they go without.

The dank shanty towns in which they live are viewed by the government as "illegal settlements" and the two million inhabitants of Nairobi's slums, of whom 43% are under the age of 15, therefore receive no state support or basic services.

Time and again in Korogocho, I encountered children who had dropped out of school to scavenge for food in the nearby Dandora dump site, a giant garbage mountain of the waste of Nairobi's entire four million inhabitants. In one test carried out on Korogocho children, more than half had blood abnormalities that signalled heavy-metal poisoning which exceeded accepted international levels. This is said to result from toxic fumes released from the garbage burned there and other chemicals leeching into the soil, water table and food chain.

In Korogocho there are few schools anyway, so the need to find food takes priority over education for most children.

"The kids go to Dandora, spend all day there sometimes, raking through the filth for things to eat or sell that might give them or their family a morsel of food," says Adan.

No sooner had he spoken than I came across a boy no more than 10 years old carrying a string of filthy chicken heads and offal he had retrieved from the dump.

"We will boil them, for a soup," said the boy, who would give his name only as Tom, before heading on his way, labouring under a filthy torn sack filled with recyclable plastic and other items his foraging had also provided.

Here in Dandora and Korogocho, alongside the resident slum youngsters, there is also a massive presence of street children who try to escape from police round-ups in the city by hiding in the shanties.

For those lucky enough to survive into adolescence, more often than not a life of crime and violence awaits them.

In Mathare I met James Mawanzia. Born and brought up in the slum for 27 years, he knows all about the pressures of growing up here. "Yes, I was what you might call a bad lad," he tells me as we skirt around the narrow, muddy, rainy-season alleyways of Mathare.

"I was a thief and stole many times, and saw friends die who were no more than boys, some shot down by the police or other gang members," James admits.

Today he works as part of a community programme aimed at helping the people of the slums, especially the young. During my visit James introduced me to 13-year-old Reuben Mbithe, who, together with his grandmother and sister, lives in a shack surrounded by others just like it, heaps of garbage and open sewers.

"I play football for a junior side," says Reuben when I ask him how he spends his spare time.

"One day I hope to play for Mathare United," he tells me, breaking into a grin at the thought of joining the professional slum-based team that has become something of a legend in Kenya.

As we leave the neighbourhood, outreach worker James points out that Reuben is "a good kid". Like so many other youngsters here in Mathare, Korogocho and Nairobi's other slums, they have as much natural talent and potential as anyone else if only they were given the chance to excel, he tells me.

"They don't need to end up in gangs stealing or scavenging to survive. It's just that places like this leave them little option."