Six days a week, Luis Enrique Herrera rode his bicycle to work, a round-trip journey of nearly 20 miles. He worked with his hands as an auto mechanic and seemed to his relatives a healthy 35-year-old man, which is why they did not feel overly worried when he had to go to the hospital. "We thought he had a common cold, something normal," said his younger brother, Gabriel Herrera.
It was 12 days ago that Luis Herrera walked into this city's National Institute for Respiratory Illnesses with a fever of more than 102 degrees, aching bones and breathing problems. Now he is isolated, uncommunicative, bedridden and breathing through a tube. His doctors have not confirmed which strain of flu he has contracted, but his family fears it is the deadly new swine virus that has virtually shut down this city of 20 million people.
"He just kept getting worse and worse and worse," Gabriel Herrera said. "His condition now is really very grave."
The anxiety over the virus has vastly altered the rhythm of Mexico City, with millions of people staying home and many of those who venture out doing so wearing masks. On Sunday, Catholic Masses across the city were canceled. One of the most popular Mexican professional soccer teams played a game in front of an empty stadium, which can seat more than 100,000 people. Mayor Marcelo Ebrard said he might have to shut down all public transportation if the crisis worsens.
The question of who contracts and ultimately dies from this virus has become a matter of central concern in Mexico. And the answers that are beginning to emerge as the death toll rises have been ominous. Relatively young adults, presumably among the population's most healthy, have been the first to succumb. All 86 people suspected to have died of swine flu in Mexico were ages 25 to 50, said an official at the Health Ministry, speaking on the condition of anonymity.
All the 15 people in Mexico City who died from the virus were 25 to 37 years old, Ebrard said in a radio interview Sunday.
The high proportion of young adults among the fatalities is one of several mysteries about this virus. The same pattern emerged during the 1918-1919 Spanish influenza epidemic, which killed at least 50 million people, and it remains unexplained in that case as well.
One theory is that the virus triggers an excessively aggressive immune response that destroys the throat and lung tissue. Young adults, with the most robust immune systems, may be especially at risk.
The greatest concentration of cases and deaths have been in Mexico City, the surrounding state of Mexico, and the state of San Luis Potosi to the north. Health Secretary José Ángel Córdova said 30 suspected swine flu cases are spread out among 17 other states.
Most of the fatal cases involved extensive lung damage, requiring doctors to prescribe mechanical breathing assistance. Exactly what caused the lung damage is not known.
Justino Regalado Pineda, an epidemiologist with the Health Ministry, said adults would be more likely to contract the flu simply because they tend to congregate more in public places, such as at their workplaces.
He speculated that one reason people have died in Mexico as opposed to the United States is that the life span of the virus could have been longer in Mexico.
After flu infections, people can develop an additional bacterial "superinfection" that could be lethal, said Brian Currie, an infectious-diseases doctor and vice president and director of clinical research at Montefiore Medical Center in New York City. Currie said it remained a mystery why people in Mexico were dying while the cases reported in the United States have been relatively benign.
"You've got to remember, this is a strain of flu nobody has seen before," Currie said.
Even though there is no known vaccine for humans for this strain of swine flu -- which combines genetic material from more common types of pig, bird and human flus -- Mexican officials have stressed that it is curable. President Felipe Calderón said Sunday that of the 1,324 patients with flulike symptoms as of Saturday, 929 have been treated and released from the hospital.
Mexican officials said there is no shortage of antiviral medication. The difference between who lives and dies seems largely linked to how quickly patients receive treatment, officials said.
"With a sickness like this, if you don't take it seriously, if you don't go to the doctor right away, it can have very grave consequences," Calderón said in a televised address Sunday.
Calderón gave a national lesson on public health, instructing people to wash their hands regularly, wear surgical masks, cover their mouths when they cough and avoid sharing food. Officials in Mexico City have handed out 6 million masks.
"Everyone, absolutely every Mexican, needs to make a special effort to avoid contacting other people who could potentially be infected with the virus," the president said.
Jorge Francisco Guzmán Suárez, a 24-year-old who died Saturday at the National Institute for Respiratory Illnesses, was initially treated by a private doctor for a stomachache, rather than the flu, his aunt, Herminia Guzmán, told the Reforma newspaper.
"We are devastated," the aunt told the paper. "The miracle did not arrive."
An outdoor market in the colonial neighborhood of Coyoacan on Sunday was a shadow of its usual self. Candelaria Villanueva, 72, a vendor of jewelry and blouses, said sales have plummeted. She was worried, she said, because her 20-year-old granddaughter recently got sick and was told by a doctor that it was "just the flu."
"I think you have to have faith in God," she said.
A double-decker tour bus was nearly empty. Bus worker Karla Yañez said people are too scared to ride.
"Everybody's inside, places are closed, the parks are closed, people don't go out," she said. "Mexico is a social place -- people like to go out and be together. The sickness has taken that away."
Staff writer David Brown in Washington and special correspondent Jonathan Roeder in Mexico City contributed to this report.