Awoman on the Red Line train hunches over in her seat, and all around her, morning commuters stare. She is utterly unremarkable except for one small accessory.

She is wearing a white face mask.

A month ago, this woman might have been just some eccentric, but these days when you see a face mask you think SARS. And you wonder, has it come to this? D.C. isn't Hong Kong or Toronto. No one in the United States has died from SARS. Still, the woman spreads a germ of doubt. Perhaps you wonder what she knows that you don't.

SARS spreads fast. It sickened 400 people and killed nine before it even got a name. In little time, the scourge spread from Asia to 17 countries, killing more than a hundred people. It is invisible. There's no known cure. It produces no marks on the skin -- no pustule, no pox, no bruise. In early stages, it looks like the common flu. Would you know if you were in a subway car with a SARS victim? Would you know if he'd given the virus to you?

Edna B. Foa, who considers herself a "courageous person," has for some time had plans to go to China at the end of the month. She is director of the Center for the Treatment and Study of Anxiety at the University of Pennsylvania, where she studies compulsive hand-washers and other people dominated by their fears.

She has canceled her trip.

"I think I'm thinking about it logically, although I also know that the probability of getting killed in a car accident is higher," Foa says. The difference is that driving feels like "it's under my control."

Epidemics can make us like animals -- we die like rats and riot like monkeys. We go wild with fear; we run dry on human decency. Fear is proportional to ignorance. When no one knows where a disease comes from, everyone is next. Horror movies and bestsellers attest to a fascination with invaders -- from across the world, from laboratories, from outer space. But with infectious disease, the invader comes from us. Friends become enemies. Lovers kill. Boccaccio once wrote of the height of barbarism: mothers abandoning children during the 14th century's Black Death.

There is the specter of dying toxic and utterly alone. There is the prison of quarantine, and even of reverse quarantine -- our desperate efforts to keep outsiders out. There is the eerie notion of a patient zero, who witlessly kills thousands, millions. There is the certainty that this is it, doom has finally arrived -- the fourth horseman has swept into town on his sickly pale horse, dragging the death cart behind him. (The Malay Mail tabloid recently referred to "the SARS curse," as if Apollo himself sent the virus, as he sent a plague upon the Greeks.) There is a feeling that God's summary judgment has arrived. This one will get us all.

There's no such thing as a fear vaccine.

SARS is especially scary nowadays, in our germ-wary world, because for a long time we have considered ourselves nearly impregnable. There are always more tests, more vaccines, more cures to save us. We see microscopic bacteria and viruses not as part of the checks and balances of the natural world, but as a rogue nation to be dismantled. It's us against them. The finicky pull out antibacterial lotion after trips on the subway. They use paper towels to open bathroom doors. There is a sense that if we're diligent enough, we can sterilize our environment.

But the virus just mutates and comes back another year.

Black Flags

In the Middle Ages, when the bubonic plague hit a town, the people ran up a black flag to warn away outsiders, says Bryon Lee Grigsby, an English professor who studies medicine in medieval literature at Centenary College in New Jersey. Nowadays, Toronto hospitals close. A conference in the city is canceled. Limo drivers reportedly won't take family members to the funeral of a Toronto SARS victim. In Hong Kong, 240 residents of an apartment complex are sent to quarantine camps. The CDC gets more than 13,000 worried calls about SARS.

Like SARS, the polio scares in this century were frightening for the disease's unpredictability, its lack of warning. Go to bed a little under the weather, wake up paralyzed.

"If you cannot tell who has it, you cannot protect yourself from being next to that person," says Naomi Rogers, a Yale University historian of disease.

What happened in certain communities was wholesale panic, and childhood pleasures were disinfected clean away. Summer schools, camps, pools were closed. Children were not allowed to leave the house to play. Siblings of children with polio were treated as pariahs. Even ice cream was implicated, so some children were denied it, Rogers says.

"In the 1916 epidemic, the epicenter of that epidemic was New York City, and New York City itself got associated as the cause and the transmitter of the disease, so that the governor of Pennsylvania set guards up along the borders of the main roads from New Jersey into Pennsylvania and refused to let anybody in who was from New York," she says. Summer resorts in Long Island put up signs: "If you are from New York City, turn around now."

During a cholera scare in Great Britain in the early 1830s, people became mistrustful of the medical community, and they attacked hospitals with cries of "burn it to the ground!" says Jo Hays, a recently retired history professor at Loyola University. When the bubonic plague hit Moscow in 1771, Hays says, some felt the wrong people were being quarantined and they rioted. They killed an archbishop.

Panic is the ouroboros serpent, eating its own tail.

During the waves of bubonic plague that swept across Europe, the rich people fled and the poor grew frantic. They tried huddling by fires. They tried powder and perfumes. Because the plague was bound up with bad smells, the very air seemed contaminated, which is why doctors wore -- along with a medieval biohazard suit made of leather robe and hood -- a beak filled with herbs or flowers. Filter out the stink and maybe you could filter out the pestilence. They were wrong.

During the Black Death of the 14th century, which was probably caused by bubonic plague, which killed almost one-half of Europe, people blamed the sin of pride, and roving bands of flagellants beat themselves to atone for the world's sins, hoping that God would forgive the human race.

They also blamed changes in the air caused by volcanoes or astrological influences. And they blamed the Jews for poisoning their wells. Outsiders have always been suspect when it comes to disease. Don't forget that AIDS was originally known as GRID: Gay Related Immune Deficiency. In France, says English professor Grigsby, it was known as the hemophiliac's disease.

"When a disease comes in, there's a desire to construct it so it's not us, it's someone else," Grigsby says. "We'd be really happy if once you got the disease, your hair turned green and shot off to the side because then we could identify you."

We rope ourselves off from the sick, from what Susan Sontag calls the "night-side of life." Sick people should look sick so we know what to avoid, so we know we're truly healthy.

This is why AIDS grates on us to such a degree, says Grigsby. "With AIDS you can operate in the land of the healthy and be incredibly ill for 10, 20, maybe 30 years now, still be able to infect people, and no one can identify you."

Sick Meaning

When it comes to making metaphor out of epidemic disease, AIDS is impressive. If you're looking to blame the victim, AIDS wins out because -- unlike SARS or the bubonic plague -- it spreads in a selective fashion, based on easily defined behaviors, like sex.

But less selective diseases can be moralized, too. Take tuberculosis, once known as consumption, which had a certain fashionability in the first half of the 19th century. It was the disease of artists and sensitive types, and the consumptive's thinness was considered attractive.

But later on, says Hays, TB came to be associated with poverty "and at that point, it lost its cachet." Poverty, after all, was linked to laziness. The afflicted were placed in sanitariums with the purpose of making them healthy, as well as "improving their moral behavior by getting them into the habits of good reliable work," Hays says.

All of this -- the assigning of blame, the association of illness with a group of outsiders, the search for meaning -- is an effort to save ourselves. Make disease a choice and you can avoid it. Don't travel to China. Don't travel to Toronto. Cover your mouth. Secretly figure that those who don't take precautions deserve what they get. This isn't the product of a peculiar nastiness of American culture or European culture. This is human culture.

Disease is confusing. When we get it, we may wonder Why me? Kafka interpreted his TB as "a sign of my general bankruptcy," according to a quote Sontag pulls from his letters in her essay "Illness as Metaphor."

Disease is confusing. How inconsistent, how impossible it seems when a strong person is weakened by illness. How convinced we are that the mind should be able to do something about the body's frailty. Photographers agreed not to take pictures of FDR's wheelchair, evidence of his polio. Disease gets all muddled up with character.

The soldier felled by a single bullet can die beautiful and brave. He dies for a cause and he dies intact. Disease corrupts us, makes us ugly, does it for no other reason than because it can. No wonder we think of the plague as evil. Consider Ebola.

The Body Snatchers There's the ghastliness of it.

That's the main draw of Richard Preston's 1995 nonfiction bestseller on the Ebola virus, "The Hot Zone." Early on, Preston tells of the death of a Frenchman living in Kenya who contracts a virus similar to Ebola. He vomits continually, and then his guts explode. He is literally turned inside out.

"The Hot Zone" is a map of fears about the plague: the virus Preston describes is incredibly infectious, it kills within days, it kills terribly, and before that it turns the person into a "zombie" with "bright red eyes," and a "sullen" personality. There is an Invasion of the Body Snatchers quality to this description -- the notion that the sick person has been invaded, has become the disease, "has been transformed into a human virus bomb."

There is smallpox, with its power to make us hideous. There is cholera, with its uncontrollable diarrhea. There is leprosy, which in the Middle Ages was considered so dangerous to the community that the newly diagnosed leper had dirt from a graveyard thrown on his head. Then he was banished, a living corpse. The diseased body becomes a death chamber.

And quarantine becomes a prison sentence. People "are afraid that they'll be hideous, that they won't be found for days, that they'll be dying alone, that they'll be unable to reach those they love," says Ann Carmichael, a history professor at Indiana University who specializes in the history of infectious diseases.

Viewing disease in apocalyptic terms is the only way to justify all this suffering. The Hebrews invented the idea of apocalypse as a way of reassuring themselves that God would vanquish in the end, that all their misery would not be for naught, says Craig Hill, a New Testament professor at Wesley Theological Seminary in Northwest Washington. Never mind that the diversity of the human population and its immunities mean that it's highly unlikely one organism could possibly get all of us. This is not about reality; it's about fear.

And SARS reinforces the notion that no corner of the world is safe. Stanley Deresinski, an infectious diseases expert at Stanford University medical school, says a professor once told him to "perceive of the entire world as being covered in a thin film of feces." No wonder we have a sense of doom.

Deresinski says SARS bears some similarities to the 1918 outbreak of influenza, or Spanish flu. "It's a respiratory virus," he says. And it may be spread in a similar fashion. The Spanish flu killed 20 million people worldwide.

How could people prevent what was everywhere?

Back then there was a children's song that went like this:

I had a little bird,

Its name was Enza.

I opened the window

And in-flew-Enza.

Cover your mouth.

A woman in Beijing last week was one of many people wearing masks to avoid infection with severe acute respiratory syndrome (SARS). Responses to the withering force of disease: Above, a Spanish flu ward at a U.S. Army hospital in 1918. Left, the AIDS Memorial Quilt on the Mall in 1996. Far left, workers remove the body of an Ebola victim in Gabon in 2001.