For little Kevin, it was a bala perdida that almost did it: the stray bullet went in through his nape and came out through his brow. That was a year ago, when he was four. The incident took place a few yards from where we now sat, in a front room that felt like a car-less garage, with its damp cement floor, and a series - almost a pattern - of scorched light fixtures along its walls and ceiling. Kevin's grandmother runs a modest line in second-hand clothes; there was a stretched wire with some coat hangers on it, and a plastic bag stuffed with espadrilles and flip-flops. The family dog, small, frazzled and elderly, was still growling at us after half an hour, even while scratching its ear with a raised hind paw.

Kevin was playing in the street when the car sped by (it never became clear to me what, if anything, the muchachos were trying to hit). At the hospital, his 20-year-old mother was told that Kevin had five minutes to live. They operated; and, after a five-day coma, a silent and unsmiling spell in a wheelchair, and a course of rehabilitation, Kevin seems to have re-emerged as a confident, even a stylish little boy.

Kevin was deeply withdrawn for months; he responded only listlessly to other children, and was indifferent to adults. When he divided his toy soldiers into good guys and bad guys, the bad guys always won.

What happened to Kevin was an accident: an accident in a very accident-prone city, but an accident. Another child, 10-year-old Bryan, will find it harder to gain the (in fact nonexistent) consolation of 'closure'. He was shot in the back by his best friend. Bryan's offence? It wasn't as if he threatened to take his football home - all he did was say he didn't want to play any more. Bryan now has a palsied gait (a slow, bobbing hop) and a face deprived of symmetry; and he looks blind too (though he isn't), because his gaze seeks nothing. Kevin, on the other hand, amiably complied when his grandmother, parting his hair and lifting his fringe, showed me the entry wound, the exit wound; they looked like vaccination scars. As we took our leave, the dog gave us an eloquent snarl: good riddance to bad rubbish. The dog, it seemed, had taken on the fear and distrust that ought to belong to Kevin.

In the forecourt of the house opposite, a fully adult male (statistically quite a rarity in this neighbourhood) was closing up his house for the night; he stared at us with frank but nonspecific hostility, all the while rearranging the contents of his crimson running shorts.

Some residents try to disguise it with fancy grilles and lattices, but most of the houses in non-downtown Cali are wholly encaged. The male adult across the street now proceeded to wall himself up in his personal penitentiary. In El Distrito, the boys rage all night and sleep all day (in their coffins and crypts); and at dusk they all turn into vampires.

We always had to be out by five - but wait. There was still time to visit Ana Milena. Some years ago her sister had been paralysed after being shot in the throat by a neighbour; she died of depression and self-starvation in 1997. Seven years later, Ana broke up with her boyfriend. So he attacked her in broad daylight at a bus stop, stabbing her in the navel, the neck, and twice in the head. Their daughter (then nearly three) stood and watched, and hid her face. She still insists that her mother was hit by a car.

Gang slang for a home-made gun is una pacha: a baby's bottle. The violence starts at once and never goes away. Kevin's scars are not at all disfiguring. He has an entry wound and an exit wound. His was easily the most hopeful story I heard in Cali. In general, you suspect, emotionally and psychologically there may be entry wounds, but there are no exit wounds.

2. La Esperanza

Occupying about a quarter of Colombia's third city, Aguablanca (Whitewater) consists of about 130 barrios; each barrio has two or three gangs, and all the gangs are theoretically at war with all the others. What do they fight about? They don't fight about drugs (ecstasy and dope are popular, but the cocaine trade is an elite activity). They fight about turf (a corner, a side street); they fight about anything at all to do with disrespect (what might be called 'eyebrow' murders); and they fight about the fight that went before (venganza operates like a series of chain letters). Yet the main fuel of the murder figures, here as elsewhere, is the fantastic plenitude of weaponry. A home-made gun costs just over £20, a hand grenade just over £12 (a hand grenade is what you'll be needing if, for instance, you gatecrash a party and get turned away). 'Guns don't kill people. People kill people,' argued Ronald Reagan. You could take this line further, and say that people don't kill people either. Bullets kill people. In Cali they cost 50 cents each, and can be sold to minors individually, like cigarettes.

Three teenage girls, acting as the representatives of a barrio called El Barandal (The Rail), advised us not to enter; but a couple of hundred yards down the road, at La Esperanza (Hope), we were casually welcomed. I asked what had made the difference, and our driver said that El Barandal was even poorer and dirtier and, crucially, fuller; there was more humiliation, more wrath, and more guns. Sara, the friendliest of the Esperanzans, had a different emphasis: 'Somos todos negros, y somos buena gente.' We're all black, and we're good people. And good people they would need to be. Every South American country has its own name for places like this. In Bogota the word is tugurio (hovel), but the Chilean version best evokes La Esperanza: callampa (mushroom). 'Whitewater' suggests a fast-flowing river, or rapids. The marshlands where the barrios sprouted up, in the 1980s, are now whitened by their own putrefaction. The endless ditch isn't deep enough to submerge the tubs and tyres that disturb its caustic mantle. Yet the egrets still consider it worth their while to paddle in it and peck in it; when they flap their wings you expect them to fly off on half-corroded stilts.

The people here are desplazados, displaced peasants, mainly from the country's Pacific coast. Cali contains about 70,000 of the displaced. Some are pushed from the land by that irresistible modern force, urbanisation; others are fleeing what may be the final convulsions of a civil war that began in 1948. But here they are, with no money and no jobs. Colombia does not provide free health care or free education for its citizens; and the first explanation you reach for here is the enormous South American lacuna - taxation. Taxation, necessarily of the rich, is not enforced. To paraphrase the former president Lleras Camargo, Latin Americans have gone to jail for many strange reasons, but not one, in the whole continent, has ever gone to jail for tax fraud.