Airline employees who report to work ill are more likely than sick passengers to spread infections such as the H1N1 swine flu virus aboard airplanes, with low-paid workers posing the greatest danger, a U.S. government expert said on Thursday.

Dr. Michael Bell, an expert on infectious disease with the U.S. Centers for Disease Control and Prevention, said flight attendants and other employees who move through aircraft can leave germs on any number of surfaces, while sick passengers could be more likely to remain stationary.

But the greatest threat could come from a low-paid airline contractors, such as cleaners, if slim wages and poor benefits make it difficult for them to take a sick day.

That individual may be just as effective at spreading infection as anybody else, Bell told a meeting sponsored by the independent National Research Council on the role that airports and aircraft play in transmitting disease.

Public health officials ask sick people not to travel and risk spreading infection to others and advise ill workers to stay home as well.

The way we help employees not to be sources of transmission is pretty complex because there's such a variation between the resources those people have, said Bell, who is an associate director for infection control at the CDC.

With contract staff in many ways being a larger part of the work force, it becomes tricky, he added.

The question of infection aboard airliners and in airports has become a major concern in the United States, where public health officials are bracing for a surge of H1N1 swine flu cases as cooler autumn temperatures take hold in the Northern Hemisphere.

The U.S. Department of Homeland Security is urging air carriers to emphasize personal hygiene among workers and passengers, particularly hand-washing, as health officials await the first delivery of H1N1 flu vaccine later this month.

Commercial airliners have air filters that can trap pathogens and prevent them from spreading through the plane's ventilation system.

China Medicine