They were not taking any chances because the country they just left has been hit the hardest by this outbreak. The disease has killed up to 86 people so far and in less than two weeks sickened 1,400 people.
"Most everybody's taking it seriously because all the schools closed, and a lot of restaurants are closed, and night clubs and everything. So, it's pretty serious," said San Francisco resident Alma Esequibel.
Mexico City has been virtually shut down by the /*swine flu*/ outbreak. Concerts, movies, theaters and schools have all been suspended until a week from May 6th. Mexico City residents like Erick Marban said they almost did not recognize it.
"I think there's a lot of panic on the streets. People don't want to go out. They prefer to stay at home and actually driving is much easier these days because people want to stay at home," he said.
They are staying home and changing other customs too, like following an advisory to not kiss on the cheek when saying hello.
"Usually, in Mexico it's very common to say hi giving a kiss. And, you can't, you shouldn't," said Harumi Cruz.
Cruz has a cousin in the hospital in Mexico City who has symptoms of the virus.
"I was scared because my baby has a heart disease and you know, this is a virus infection and I didn't want the virus to get into his heart, and it's very scary," she said.
In San Francisco's Mission District Sunday, people with relatives in Mexico City were also scared.
Hector Torres was supposed to catch a flight to visit family in his native Mexico City Monday. He cancelled it and instead watched a soccer game on television where there were no fans in the stands.
"It looks very bad. When you see the news and all the people is wearing masks it's amazing," he said.
Saint Mels Catholic School in Sacramento County will be closed until at least Thursday while health officials look at whether a seventh grader may have contracted the disease.
"I cannot tell you when it will be confirmed or disproved. All I can tell you is that it is on its way now and I know the CDC will do its best to run the tests and give us the results as quickly as they can," said Sacramento County Health Officer Glennah Trochet.
So far seven confirmed cases of swine flu have been reported in Southern California. On Sunday, the number of U.S. states with confirmed cases grew to five.
The U.S. government has not placed any restrictions on traveling to Mexico, but some airlines are now allowing passengers to reschedule their flights to Mexico City at no extra charge. This weekend, health officials in California have ramped up testing for the virus in hospitals and clinics, expecting that there will be more cases appearing in the state soon.
Governor Schwarzenegger has scheduled a news conference at 11 am on Monday to discuss California's response to the swine flu outbreak.