In the face of such a devastatingly infectious disease, prevention is key.

Hong Kong's proximity to the mainland and its role as a global nexus puts it in a particularly delicate position. Visitor health screening should be strongly considered and implemented judiciously. As well, hospitals and clinics should be bolstered with H7N9 testing; health care personnel placed on high alert for suspicious cases and stockpiles of antivirals be updated and assured.

There has been no human-to-human transmission, the advent of which is crucial to a pandemic alert.

Avian influenza, or bird flu, is an infectious disease of birds caused by viruses of the same family that causes the "flu" in humans. They are, however, not identical and have important genetic distinguishing features. Normally, avian influenza viruses do not infect humans. But every now and then, these viruses and human influenza A viruses converge in a host (fowl or mammal) and reassemble into a new strain. This drastic form of mutation is known as "antigenic shift". The new strain is more lethal and very transmissible due to the nature of poultry farming and an interconnected world. Worse, due to their novelty, these viruses are usually not covered by the annual vaccine profiles. In the past decade, one such dreaded strain, H5N1, alerted the world to just how catastrophic an antigenic shift could be. Since its first outbreak in Hong Kong in the late '90s, it demonstrated widespread re-emergence in 2003 and 2004, spreading from Asia to Europe and Africa, has become entrenched in poultry and continues to claim lives.

Generally speaking, human flu is caused by a relatively simple (structurally) ribonucleic acid (RNA) virus. It is spread via contact and airborne droplets and thus commonly affects the respiratory system (nose, throat, lungs). The body's immune system then mounts a response to combat the virus and regenerate dead infected cells. From the time of exposure it takes about one to two weeks to recover from a bout of flu for healthy individuals. In the very young, elderly and those with compromised immune systems symptoms can lead to severe disease and potentially, death.

Human influenza is found in virtually every part of the world. According to the World Health Organisation, human influenza is thought to be responsible for between 250,000 and 500,000 deaths every year globally. Three types of human influenza viruses have been identified: A, B and C. Of these, influenza A appears to have the greatest impact on humans. Influenza A viruses are further sub-classified according to two different proteins found on the viral surface: haemagglutinin (H) and neuraminidase (N), giving rise to names such as A(H1N1), A(H3N2) etc. These many subtypes are due to the relatively high mutation rate of influenza viruses; attributable to their simple structure and genetic make-up. This form of mutation is frequent and known as "antigenic drift". It forms the basis for annual vaccinations because a new vaccine is needed to match the new, slightly different virus.

This differs from "antigenic shift", the less common, but more radical mutation, whereby a new subtype emerges from the combination of two different strains of viruses, such as avian and human influenza viruses. Like the A(H5N1), antigenic shift mutation was the main driver for the existence of the A(H7N9).

First, there is the individual response. Basic hygiene practices and food safety measures are imperative in preventing the transmission of infectious diseases. Frequent and effective hand washing, especially before, during, and after food preparation and consumption should be instilled in every household and business outlet. Be sure to cover your mouth and nose when coughing or sneezing. And avoid unnecessary close contact with people suspected of an influenza-like illness and, in the case of avian flu, live or uncooked poultry as well.

Secondly, there is the community/global response. Enhanced surveillance, open communication, international collaboration, epidemiological procedures and public disclosure are absolutely vital. Recent prompt announcements from China's health authorities are applauded, but more details would certainly be appreciated.

Hopefully, harsh lessons learned from the Sars outbreak continue to reverberate within the corridors of power.