But Secretary for Health, Welfare and Food York Chow Yat-ngok was not able to say what cell types are susceptible.

"According to experts' analysis, some recent human cases have involved not only infection of the respiratory tract, but other organs are being destroyed as well," Chow said after a business lunch.

If it is true that the virus may not only infect the lung, Chow said, then the sole dependence on inhalable forms of antiviral drugs such as Relenza that are used to treat both influenza A and B will not treat the disease effectively.

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"For the sake of prevention, we need to depend on different drugs," Chow said.

"Our medical staff must realize the disease might not be limited to respiratory tract infection. We will have to conduct various tests."

Chow admitted he could not give a definitive answer when asked what part of the human body the virus attacks first, but it appeared to start in lung cells.

The exact means of how the disease develops in the human body remains a medical puzzle.

According to research from the Erasmus Medical Center in the Netherlands published in the journal Science on April 21, the H5N1 virus may cause severe lower respiratory tract disease and severe pneumonia in humans.

"However, the cell types in the lower respiratory tract to which the virus attaches are unknown in both human and experimental animals," the scientists wrote.

The persistence of the outbreak of avian flu in Asia, which has subsequently spread to Russia, The Middle East, Europe and Africa, has been blamed in part on migratory birds.

A separate Science article said surveillance in the mainland from 1999 onward indicated that H5N1 was endemic in domestic birds in the region and that multiple genetic lineages of the virus are circulating.

The poultry trade and mechanical movement of infected materials are likely modes for spreading the flu in general, the article said.

Given the proximity of China, Chow said "the Hong Kong government has been talking to the mainland authorities regarding a joint exercise, particularly about contingency drills for infectious diseases."

He added: "We are looking at the best way to do that and how we can have an exercise that will have cross-border activities at the same time," without giving a specific schedule.

The latest H5N1 outbreak began ravaging Asian poultry stocks in late 2003 and has killed at least 113 people worldwide.

Most human cases have been linked to contact with infected birds. Experts fear the virus will mutate into a form that can spread easily among humans, sparking a pandemic.