Water curls through the Fortaleza slum: sewer water. It drains between shacks made of sticks and mud, and is pretty when it catches the sun. It gathers in ‘Lagoa Porangabussu’, which is not a lake but a cesspool where the locals swim. My companion, an American medical student, and I sidestep boys shooting marbles, a sulky rooster and Renato the transvestite, who is shaving his legs. We meet a naked girl soaking in a metal tub, a man on horseback selling cow livers from wooden saddlebags, a toddler drinking thick black coffee, a skinny, nervous dog, and a fellow no more than two feet tall, who is mostly chest and smiling face. ‘Polio,’ my companion says. He has come from the plague, leprosy and typhus wards at the Infectious Disease Hospital, where there is a man with the largest foot he has ever seen.

Cholera is expected soon in Brazil. In the spring of 1991 I was in upstate New York. As the disease spread from Peru to Ecuador, from Colombia to the Amazon, I tracked it on a map as if it were weather. I studied its history. In the Egyptian cholera epidemic of 1831, 13 per cent of the population of Cairo died. Twenty-four years later, in Rio de Janeiro, 102,000 died in a single year. The cholera epidemic that devastated Cairo had originated five years earlier in Bengal, spread to southern Russia, followed Russian troops into Persia, Turkey and the Baltics, travelled by ship to England and Ireland, crossed to Canada and torn through North America. This year’s outbreak in South America, where the disease had not been seen for a century, began somewhere else thirty years ago.

I called tropical disease specialists in Rio, public health experts in Atlanta and officials of the World Health Organization. A professor of medicine at the University of Virginia thought he knew where in Brazil cholera would strike first: Fortaleza, a north-eastern coastal city of two million. If the north-east were a separate country, he said, it would be the poorest state in South America after Bolivia. The people already suffered from diarrhoeal diseases at rates similar to those in Bangladesh. I set off for Fortaleza, a city I had not heard of before cholera.