By Matthew Price
BBC News, Mexico City
Some Mexico City pharmacies have sold out of face masks
The masks begin to itch after a while.
They get sweaty as the day warms up.
Plenty of people across Mexico City are wearing them, though, on the buses, or in their cars as they drive down the highways to work.
A newspaper seller wearing a mask walked among the traffic at one intersection.
He held up the front page to passing motorists: "103 dead," said the headline. Since then, a health minister has revealed that the toll of suspected swine flu deaths has risen to 149.
Plenty of people are still moving around here but, by Mexico City standards, rush hour is quiet. They know they are at the centre of a feared global outbreak.
At one of the hospitals in this sprawling city there was no sense of chaos. As the sun rose, staff began turning up for another day.
Some came already wearing surgical masks. Others put them on as they approached the building.
None of them would give even a quick comment as they went in. It appears that medical staff have been asked not to talk to reporters.
One man did talk, though.
We have no way of verifying his story, and he did not want to be identified, but he told us he was a low-level worker in the hospital.
"The third floor is where the outbreak is centred," he said, through a translator, as his cigarette burned down slowly in his right hand.
He wore a blue mask pulled down just below his mouth.
"A total of 30 people have died, some of them are staff. Including the first nurse to work with a patient with the virus."
A little further up the road, a group of relatives sat on the steps outside the hospital - most in masks. One man, Ernesto Salinas, was wearing a white plastic builder's mask.
"I'm worried," he said. "Not just for me, but that this could spread panic across this country."
No masks left
There is no sign of that yet.
But the authorities are clearly concerned enough to close many public buildings.
A top-flight football game that had sold out was held behind closed doors - the authorities do not want people mixing in large crowds here, in case that helps the virus spread.
At the bus station, however, plenty of people were queuing up together. One man, not wearing a mask, shrugged when asked about the risk: "I haven't got flu. I'm not worried."
A woman waiting behind him said she had not been able to buy a mask at the pharmacy. There were not enough to go round.
Still, she did not seem too concerned. "We have to continue as normal," she said. "The president said in his speech yesterday we should stay calm and do as we normally do."
The big unknown here is how dangerous this virus really is. The authorities say they are taking all the precautions they can - and need to - take.
The mayor of Mexico City has said that if things do worsen, he will shut down the entire public transport system.
That would, in effect, bring this city of more than 20 million people to a halt.
For now, Mexico is at the mercy of a virus that no one yet fully understands.