Public health officials on Thursday had not advised canceling large-scale events unless they were specifically tied to an institution or location with a laboratory-confirmed case of the illness. They urged people to stay home if they are sick.
Organizers of the May Day rallies, which have drawn thousands and even hundreds of thousands of people in recent years, said they would look to recommendations from public health officials about whether to cancel or modify the events.
"We're monitoring the situation to make sure that anything that is going to be conducive to the health and safety of communities is observed," said Clarissa Martinez, a director for the National Council of La Raza.
The rallies come as illegal immigrants are being blamed on some conservative blogs and talk shows for spreading swine flu in the U.S. The outbreak is believed to have originated in Mexico, where there are 168 suspected deaths from the disease, before spreading to at least 10 other countries, including the U.S.
The only confirmed U.S. swine-flu death was of a Mexican toddler whose family was visiting relatives in Texas; many reported cases were among U.S. citizens who vacationed in Mexico.
"For people who like to blame Mexicans, they are going to blame us for everything no matter what," said Jorge Mujica, a labor union activist and organizer for Chicago's immigrant rights march. "We are not going to pay attention to that."
For most rally organizers, swine flu was secondary to promoting immigration reform, including pathways to citizenship for an estimated 12 million illegal immigrants - hopes buoyed with Barack Obama in the White House and a Democratic-controlled Congress.
More than 1 million people marched in cities across the country in 2006, when some in Congress were pushing for tougher laws against illegal immigrants. Although turnout at the marches has dropped steeply since then, organizers say their mission remains the same.
"It's important for us to continue the fight," said Margarita Klein with Workers United in Chicago, adding that union workers had been preparing for two months for Friday's event.
Crowds on Friday were expected to be around the same as last year. In Chicago, which has had the nation's largest marches in recent years, about 15,000 participated in 2008. That's a dramatic drop from 2006, when more than 400,000 took to the streets.
Thousands also were expected at events Friday in Houston, Los Angeles, Miami, New York, Seattle and other cities. Thousands also were expected at events Friday in Houston, Los Angeles, Miami, New York, Seattle and other cities. Health officials urged participants to use common sense, including washing their hands and avoiding people who are coughing or sneezing.
"Of course, anyone who doesn't feel well should stay home," said Melaney Arnold, a spokeswoman for the Illinois Department of Public Health. She added that she doesn't think the march should be canceled.
Some schools have closed because of the swine flu outbreak, and U.S. and Mexican officials have been urging migrant workers to take health precautions and get medical care if they feel sick.
Migrants Warned About Flu
As migrant workers from Mexico begin their journey north to take summer jobs in fields and construction sites across the U.S., public health officials and others are fanning out to intercept them at food lines and churches in hopes of stemming the spread of deadly swine flu.
Mexican consular officials, social service organizations and health authorities are handing out Spanish-language fliers with information on swine-flu symptoms and prevention tips. They are sending out mobile health care crews in buses or vans. And they are urging workers who feel sick to go to the hospital or a free clinic.
If seasonal labor trends mirror last year's, the number of agricultural workers in the U.S. will grow to about 700,000 by June and peak at 828,000 by September, according to U.S. Department of Agriculture figures. It is unclear how many are from Mexico.
A farmworkers union plans to meet Mexican legal guest workers as they arrive in the U.S. with a medical van, where they can get screened for flu symptoms and learn prevention tactics. The workers will later pick vegetables on North Carolina farms.
Pastors and health care workers also are hoping to reach tomato pickers near the Florida town of Immokalee and field hands harvesting asparagus in Michigan.
The Mexican consulate and local health organizations in California are mounting a prevention campaign that will send buses to isolated communities where there is no doctor. Health care workers aboard the buses will talk to people about swine flu and where they can find medical care.
The traveling population of poor farmworkers, day laborers and construction workers poses a challenge for authorities, who say it can be difficult for people to wash their hands or go to the hospital if they lack running water or fear deportation.
"People are constantly coming here from Mexico and migrating back and forth," said Dr. Edward Moreno, director of public health in Fresno County, some 440 miles north of the Mexican border. "That means that people may not have a land line, hot water or Internet access, and no regular doctor."
Alfredo Mendoza, 24, of the Mexican state of Oaxaca, crossed the border two weeks ago to work with his family pruning California's vineyards.
"I feel healthy, so I'm just washing my hands a lot and keeping my mouth covered, and not leaving the house other than to work," he said. "People aren't too freaked out about the flu here yet. I just feel lucky that I left Mexico before it got really bad down there."
2009 H1N1 Flu Outbreak Map:
This is a map depicting confirmed and suspected cases of the 2009 H1N1 outbreak, with contributors from all over the world, from a variety of backgrounds including health, journalism, technology.