Sunstroke – real cranium-boiling, bone-melting sunstroke – is a true ordeal. But ordeals can sometimes be fruitful.

Guapi is a little town on the Pacific coast of Colombia. It's a particularly remote and isolated part of the world. A steamer calls in twice a month; there's a vertiginous track winding through jungle-covered mountains that will take you to the city of Cali inland in a week or so, if mudslides haven’t closed it off; and the occasional small plane makes it to the grassy air strip outside town, to be met by a ‘bus’ that is simply an old truck chassis with a steering wheel. The few passengers pile on to the flatbed with their bags, while kids run alongside as it bumps into town.

    Guapi is in the delta of a river, amid a maze of channels threading through the rainforest here. There are Indian villages upstream. There are also primitive goldmines, consisting of not much more than a man and a pump. In the one hotel in town, run by two Swiss brothers who have been there more than four decades, I hooked up with Gus, a vagrant gold prospector from South Carolina who was heading up one of the many tributaries, and offered to take me to an Indian village on the way.

    We set off early in his launch, accompanied by his pilot and sidekick Ramon, who knew the area well. At first all was well. We wove a route through the labyrinth of brown channels and streams as the sun came up and burned the mist off the forest. We found the Indian longhouse Gus had in mind, a half-built platform in the jungle, partially covered with a thatched roof. It was around eleven that I started to feel strange. Glassy, a bit dizzy, nauseous.

    I thought I was used to the Colombian sun, but there's nothing like an open boat under an equatorial noon. I realised something was up, and bought myself a straw hat at a kind of trading post we stopped at a few miles further on. But it was too little too late. My insides turned to liquid. I lurched to the outhouse at the back of the store. Fortunately I had a diarrhoea pill in my bag, and managed to keep it down long enough before retching. As we headed off downriver again, I began to tremble. I pulled on my coat, feeling cold in the humid hot jungle. My jaw chattered uncontrollably. Every so often the side of the launch would swing up at me, I’d grip it and try to heave the contents of my belly overboard; nothing much came out.

    Meanwhile the boat was having problems of its own. The outboard died a couple of times, and Ramon spent a while with the engine cowling off, laboriously coiling a piece of cord round the spindle and tugging it. The engine would choke, emit a plume of blue smoke, and die again. When he eventually got it going, it ran another ten minutes then spluttered out once more. On it went like that through the afternoon. We’d gurgle and speed along at maybe ten knots, cutting an easy furrow through the brown water, then a sudden quiet, the hiss of the water against the sides, and we’d be drifting.

    We went back by a different route. We slipped out of the river-system and into the sea. Ramon said it would be quicker if we hugged the forested shore then went up the river-mouth that Guapi was on. It might have been had the engine not kept cutting out. By late afternoon when we’d all had our turns at heaving the cord only to hear it cough and die yet again, Ramon spent a long time actually trying to fix it, rather than just restart it. But with only one screwdriver and a spanner it was a challenge. For an hour we drifted gently away from shore. An island – the Isla Gorgona – slowly grew on the horizon, until it was so close we could see the individual trunks of the trees on its shore. The river was pushing us out to sea, all those millions of cubic metres of water spilling out of the rainforest.

Ramon noticed how far out we’d come – I discovered later that this island was some twenty miles from land – and with a new fervor we all set to trying to fire up the motor again. By the time it eventually took, we could hear the surf against the island’s rocks.

    In intermittent spurts we fought the current, and made it back into the estuary. By now we had travelled a lot further than planned, and with the last drops of fuel reached a makeshift mine where they had petrol that they agreed to sell us.

    Night had fallen. In the way these things sometimes go, in the darkness the engine ran more smoothly, and the pilot knew his way back no problem. In an hour we were home.

    I was shivering worse than ever. I stumbled from the dock to the hotel, through a night pricked with the light of the town’s kerosene lamps, hissing and trilling and throbbing with jungle insects, whooping with frogs, and climbed into my down-filled sleeping-bag intended for freezing nights in the mountains. One of the Swiss hoteliers took my temperature – one hundred and six – and said I had cholera for sure. I tuned out. The visions and stories in my head were too compelling and powerful. It was a fever like I’d never had. When someone came to check on me I must have asked for another sleeping-bag, because when I came round next morning, I found I’d stuffed my own bag inside another, in order to get warm. I woke in a bloodbath of sweat.

    It was a couple of days before I was on my feet again.

    Where does a novelist get started? There's a scene in The Lost City that has nothing to do with sunstroke – it's more a case of light-deprivation, and starvation, in the continual murky twilight of the cloud forest – but I know it was inspired by that time on the hot anvil of the equatorial sun. To me, that scene is pivotal to the book. I didn't know I had been writing towards it until I got to it; nor that I'd been writing myself away from the memory of the thirty-six hours of the highest fever I’d ever known. Still, I'll never get on a boat without a hat again.