Alarmed by the spread of a new swine flu virus, airports around the world have rushed to install temperature scanners to pick out the sick, but the microbe is proving too clever for modern technology.
Experts say an infected person can easily pass through these heat sensors without detection as the incubation period for influenza ranges anywhere between one and three days.
"The scanners won't pick up everyone (with flu), especially if they are too early in the infection … People who have been infected very, very recently wouldn't show up on the scanner," Mark von Itzstein, director of the Institute for Glycomics at Griffith University in Queensland, Australia, told Reuters.
"You can imagine somebody who is just infected boards the plane in Singapore and heads toward Hong Kong. There would not be enough time for the apparatus to pick it up because he would not have developed significant fever."
The new H1N1 swine flu virus, which has killed 159 people in Mexico, has been found in the United States, Canada, New Zealand, Britain, Israel and Spain, but there have been no deaths outside Mexico so far.
Asian airports, used to looking for H5N1 bird flu and SARS sufferers earlier in the decade, have dusted off their scanners for this latest crisis.
But experts say these cannot help until the person has a fever, and an infected person can spread the virus around by coughing and sneezing a full day before the onset of symptoms.