Hot hot heat. Taxi engines buzzing and horns blaring. Kids selling packs of gum to tourists, a bored Mariachi band knocking out a Mexicana version of Hotel California for the 100th time of the day while a donkey, painted as a zebra, looks on. Welcome to Tijuana.

We were halfway through the trek when the first sign of trouble arrived. I looked down at my boot. A slug uncoiled, extended an inch and, moving like a slinky, began climbing. Flicking it off, I spotted another on my sock. As I went to launch it into jungle oblivion, it vanished through the cotton. Panicking, I pulled the sock down to find, clamped to my flesh, a leech.