Sight is the sense we most fear losing. To create a world with an inexplicable pandemic of blindness, as Portugal's leading novelist Jose Saramago does in this novel, is to play with one of our deepest preoccupations.

By the time the measures vainly attempted by the authorities are exhausted, the entire human population - bar one woman, wife of the ophthalmologist who first diagnosed the contagion - is blind. The loss of control is therefore total. There is no expertise, no hierarchy, no politics, no electricity, no water. In such a world, nothing can be assumed. Garbage and sewage collect on the streets and food supplies are endlessly fought over. Under these extreme circumstances, Saramago asks: what constitutes a human being?

His central characters are as anonymous as those in a Becket play: "the man who first went blind"; "the boy with a squint". Initially connected by the coincidence of attending the ophthalmologist's surgery, they become quarantined in a former asylum. But instead of entering the hell of Sartre's Huis Clos, they enter one that has been imposed by the savagery of their military wardens and the gangs of thugs who terrorise the wards.

Their attempts to elect spokespersons give way to a more integrated kind of democracy, in which each has a voice and must be heard. Ironically, the one sighted woman acts less as a leader than as a servant, endlessly taking measures to safeguard the rest. This involves her in both saving and taking lives, performing both menial tasks and desperate acts.

The novel draws upon a resonant literary history of blindness, from biblical contrasts with the light within to familiar proverbs. But, since this is a work by Saramago, paradox is the order of the day. Only those who are seen can see; the doctor's wife loses her sense of sight, as she loses her sense of identity.

Other Saramago characteristics are also heavily present. A "dog of tears", Charon-like, guides the wanderers through the hell of persecution, for life and art are alike a journey. Then there is a writer who cannot cease writing, even though he cannot read his own words. Saramago is a past master at creating societies so suddenly traumatised that they have to discover again what living together can mean. The sense of dislocation in his work is enhanced by our not knowing any of the "when, where, who and why" questions that concern journalists and editors (as Saramago was, in a previous career). Having dispensed with continuity and plot, his books are free to explore philosophical issues. His characters work through the alterations that afflictions and restrictions bring to our sense of humanity, and ponder what happens "when we are all equal regarding good and evil". Finally, it is only by finding the right questions that we may receive the right answers. Saramago repeatedly undertakes to unite the pressing demands of the present with an unfolding vision of the future. This is his most apocalyptic, and most optimistic, version of that project yet.

Translated by Giovanni Pontiero, with revisions by Margaret Jull Costa. Harvill Press, pounds 8.99